chore: regenerate README (#805)

This PR was generated using Autosynth. 🌈


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

```
2021-01-15 09:45:19,444 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-15 09:45:20,296 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

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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": "90255ea7a1cc70ba4f4ab48551c509f503981540"
+        "sha": "4a2cef87f812bfd2e718d7c0e55e593ad9433c5a"
       }
     },
     {

README.md

@@ -51,12 +51,12 @@ compile 'com.google.cloud:google-cloud-spanner'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner:3.3.0'
+compile 'com.google.cloud:google-cloud-spanner:3.3.1'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.3.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.3.1"
 ```
 
 ## Authentication