chore: regenerate README (#876)

This PR was generated using Autosynth. 🌈


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

```
2021-02-16 14:56:36,897 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-02-16 14:56:38,474 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/6601876f-4193-4e02-be31-8c69bfbabd58/targets

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

Author: yoshi-automation

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "5af36739532037360dfd504a4a0988562550526c"
+        "sha": "3f5351df6d13bd3bc3639e8a75e24ae924a4cfec"
       }
     },
     {

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>16.3.0</version>
+      <version>16.4.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>