chore: regenerate README (#1408)

This PR was generated using Autosynth. 🌈


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

```
2021-09-06 02:54:36,912 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-09-06 02:54:38,215 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/7bac4f1b-48d2-490a-ba08-9e22df656cb6/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": "adc3cc1648b326802c9219efd0a056a76d198852"
+        "sha": "ca6808cc76a01167d3167faef657fb783ff459f5"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "9a238a0623879c3de129a376c6085d4a862f6eb9"
+        "sha": "ad7fd76e17bf9494a3e47ff9cf445f61564432e0"
       }
     }
   ]

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>6.12.1</version>
+  <version>6.12.2</version>
 </dependency>
 
 ```