chore: regenerate README (#810)

This PR was generated using Autosynth. 🌈


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

```
2021-01-17 23:42:11,856 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-17 23:42:12,881 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/a612e585-e096-4cad-b6bc-94f513794104/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": "77910a04e0fa42c90064fd533b6c13fe0372fb1e"
+        "sha": "5f3a67ad3a5213fa9fca977ebe4abb50ccd3f593"
       }
     },
     {

README.md

@@ -38,7 +38,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner</artifactId>
-  <version>3.3.0</version>
+  <version>3.3.1</version>
 </dependency>
 
 ```