chore: regenerate README (#783)

This PR was generated using Autosynth. 🌈


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

```
2021-01-07 05:06:13,296 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-07 05:06:14,249 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/ea9f42f1-c195-4047-a56b-19b64593653b/targets

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Author: yoshi-automation

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "df52e9d755ed8425e298ccccc5585dd8185f1a46"
+        "sha": "9b94c6ef54776fdb8868acf04e371599b7500d57"
       }
     },
     {

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:16.2.0')
+implementation platform('com.google.cloud:libraries-bom:16.2.1')
 
 compile 'com.google.cloud:google-cloud-spanner'
 ```