chore: regenerate README (#792)

This PR was generated using Autosynth. 🌈


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

```
2021-01-12 02:12:49,470 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-12 02:12:50,365 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": "fa353b35d20b6e44e6870ed86f5c55ed843ef32c"
+        "sha": "e02e5a7d95c0e92d9f13640dd2afe5b899f4e56d"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "41a4e56982620d3edcf110d76f4fcdfdec471ac8"
+        "sha": "16ec872dd898d7de6e1822badfac32484b5d9031"
       }
     }
   ]

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:3.2.1'
+compile 'com.google.cloud:google-cloud-spanner:3.3.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.2.1"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.3.0"
 ```
 
 ## Authentication