chore: regenerate README (#1076)

This PR was generated using Autosynth. 🌈


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

```
2021-04-20 05:36:27,518 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-20 05:36:28,912 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/24d40aa4-b45b-403a-9cc7-7c335d670ef0/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "7992342bffc273ad8249e7564ae9ef51764bf83c"
+        "sha": "844f0fe5308797105f2de876290f40fb230c4fa6"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "1657dd8c79a82b3fe9dde3ff6e0eda3d47fb117f"
+        "sha": "ff6b759fcb58f216f62b48d5be0bb85fafbc3bb2"
       }
     }
   ]

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:6.2.1'
+compile 'com.google.cloud:google-cloud-spanner:6.3.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.2.1"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.3.0"
 ```
 
 ## Authentication