chore: regenerate README (#1389)

This PR was generated using Autosynth. 🌈


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

```
2021-08-26 00:32:06,163 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 00:32:07,352 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/72273d6f-efcd-4b7e-86e0-f7cf2d9e5305/targets

- [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "fb3cf03fb458b8b294602bd0c07d946907a66288"
+        "sha": "269e74edb514eca0f2c686f5ba4498104a00f50b"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "63cff634aabb85854caa511c5837ea6b45f42b4b"
+        "sha": "31728d872f50e439ded2f67fa8d67955b26a4e35"
       }
     }
   ]

README.md

@@ -56,13 +56,13 @@ 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:6.12.0'
+compile 'com.google.cloud:google-cloud-spanner:6.12.1'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.12.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.12.1"
 ```
 
 ## Authentication