chore: regenerate README (#1236)

This PR was generated using Autosynth. 🌈


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

```
2021-06-08 01:48:19,003 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-08 01:48:20,392 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/614ddf95-1691-4016-8144-81a77c938b05/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": "3d4b48df2f1fd08a46113d05e37607177aebf6c3"
+        "sha": "1766c93aebf25287a47d1e04c011bb00b64f5594"
       }
     },
     {

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:6.5.0'
+compile 'com.google.cloud:google-cloud-spanner:6.6.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.5.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.6.0"
 ```
 
 ## Authentication