chore: regenerate README (#940)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-03-09 04:08:14,544 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-09 04:08:15,467 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/19317c99-3ffc-4b4c-a669-dcbd846c2c85/targets

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.github/readme/synth.metadata/synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "9c3d4f621023546466d79bec1c5da83e2dbdc84a"
+        "sha": "55fa0ccca4faf44da8f9a3553ab4b35574c14830"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "52204b292dacd3f907da614db606ed6eac8f865d"
+        "sha": "ef9d8ee2abf22c8f4d900075444e24f5a444eae3"
       }
     }
   ]

README.md

@@ -235,6 +235,7 @@ has instructions for running the samples.
 | Async Runner Example | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/AsyncRunnerExample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/AsyncRunnerExample.java) |
 | Async Transaction Manager Example | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/AsyncTransactionManagerExample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/AsyncTransactionManagerExample.java) |
 | Batch Sample | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/BatchSample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/BatchSample.java) |
+| Create Database With Version Retention Period Sample | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/CreateDatabaseWithVersionRetentionPeriodSample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/CreateDatabaseWithVersionRetentionPeriodSample.java) |
 | Create Instance Example | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/CreateInstanceExample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/CreateInstanceExample.java) |
 | Custom Timeout And Retry Settings Example | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/CustomTimeoutAndRetrySettingsExample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/CustomTimeoutAndRetrySettingsExample.java) |
 | Get Commit Stats Sample | [source code](https://github.com/googleapis/java-spanner/blob/master/samples/snippets/src/main/java/com/example/spanner/GetCommitStatsSample.java) | [![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/java-spanner&page=editor&open_in_editor=samples/snippets/src/main/java/com/example/spanner/GetCommitStatsSample.java) |