chore: regenerate README (#1383)

This PR was generated using Autosynth. 🌈


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

```
2021-08-25 00:16:26,215 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-25 00:16:27,354 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/a236642a-5f41-453a-9b7c-08f991877451/targets

- [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "41b39bc31355b3e6caaf995f416eb73f0d56dd87"
+        "sha": "fb3cf03fb458b8b294602bd0c07d946907a66288"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8032ab5a32471d85ece8fc261df16cc40fbfa4fb"
+        "sha": "63cff634aabb85854caa511c5837ea6b45f42b4b"
       }
     }
   ]

README.md

@@ -41,7 +41,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner</artifactId>
-  <version>6.11.1</version>
+  <version>6.12.0</version>
 </dependency>
 
 ```