chore: regenerate README (#701)

This PR was generated using Autosynth. 🌈


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

```
2020-12-10 03:46:15,512 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2020-12-10 03:46:16,423 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/6fa03604-75eb-4dc4-972a-1aad7ba05cc9/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": "6d45a465f77fef4d9d9d404fd6382d3e259415d5"
+        "sha": "e3289bdf1f5c723c88f4e719c4a7a15f5d131556"
       }
     },
     {

README.md

@@ -45,11 +45,11 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner:3.0.5'
+compile 'com.google.cloud:google-cloud-spanner:3.1.0'
 ```
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.0.5"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.1.0"
 ```
 
 ## Authentication