chore: regenerate README (#1245)

This PR was generated using Autosynth. 🌈


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

```
2021-06-10 15:10:17,348 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-10 15:10:18,897 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

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Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "87826b526f0d2b6ca27c0bde55f3738dbbdacfef"
+        "sha": "5afcdef967985ee9a4773005a2484f3b57d76ed5"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "2430f8d90ed8a508e8422a3a7191e656d5a6bf53"
+        "sha": "5cae043787729a908ed0cab28ca27baf9acee3c4"
       }
     }
   ]

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:6.6.0'
+compile 'com.google.cloud:google-cloud-spanner:6.6.1'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.6.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.6.1"
 ```
 
 ## Authentication