chore: regenerate README (#801)

This PR was generated using Autosynth. 🌈


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

```
2021-01-14 22:21:40,361 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-14 22:21:41,335 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/98fda3c6-0507-4f49-9189-4131c8076d8f/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "f0beabb228a4f555e1bcb1817a14e8074a54ef8c"
+        "sha": "90255ea7a1cc70ba4f4ab48551c509f503981540"
       }
     },
     {

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:16.2.1')
+implementation platform('com.google.cloud:libraries-bom:16.3.0')
 
 compile 'com.google.cloud:google-cloud-spanner'
 ```