chore: regenerate README (#986)

This PR was generated using Autosynth. 🌈


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

```
2021-03-18 00:02:30,147 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-18 00:02:31,101 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/0f5f5ae9-dd30-485d-bcce-e4841fe91bc0/targets

- [ ] To automatically regenerate this PR, check this box.

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "13c297b210939a04cab8a8fd547bb34e4d549da7"
+        "sha": "693fe5d4df3d279edb8f6f7f9879366980fd81d8"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "94686b06034ff6e11217236fcaa217ec6e24bc39"
+        "sha": "78437c732a60c64895778697b078497b0988346c"
       }
     }
   ]

README.md

@@ -45,7 +45,7 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:19.0.0')
+implementation platform('com.google.cloud:libraries-bom:19.1.0')
 
 compile 'com.google.cloud:google-cloud-spanner'
 ```