chore: regenerate README (#1391)

This PR was generated using Autosynth. 🌈


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

```
2021-08-26 03:33:01,408 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-26 03:33:02,549 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/59e1a051-1226-44d1-8d46-bdc0fdd7bce4/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": "269e74edb514eca0f2c686f5ba4498104a00f50b"
+        "sha": "175547866684ee5639b4b598d48321e156a5998e"
       }
     },
     {

README.md

@@ -41,7 +41,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner</artifactId>
-  <version>6.12.0</version>
+  <version>6.12.1</version>
 </dependency>
 
 ```