chore: regenerate README (#739)

This PR was generated using Autosynth. 🌈


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

```
2020-12-14 12:56:30,153 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2020-12-14 12:56:31,121 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/32a7f03b-dcf8-4d69-b375-3e98baebaaa5/targets

- [ ] To automatically regenerate this PR, check this box.

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "bb5a822580f91b4180fc5c0c87d8c76a459214e8"
+        "sha": "1de6c28ccea569cc8a4e8dc33e318bafb4211539"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "352f25621c28d2fa0784984ea0ed5ea697aa1525"
+        "sha": "996775eca5fd934edac3c2ae34b80ff0395b1717"
       }
     }
   ]

README.md

@@ -38,18 +38,18 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner</artifactId>
-  <version>3.1.1</version>
+  <version>3.1.2</version>
 </dependency>
 
 ```
 
 If you are using Gradle, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner:3.1.1'
+compile 'com.google.cloud:google-cloud-spanner:3.1.2'
 ```
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.1.1"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.1.2"
 ```
 
 ## Authentication