chore: regenerate README (#960)

This PR was generated using Autosynth. 🌈


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

```
2021-03-15 23:54:55,074 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-15 23:54:56,043 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/36571b4b-aa3a-4bee-aab9-01c2e132fdc5/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": "55fa0ccca4faf44da8f9a3553ab4b35574c14830"
+        "sha": "13c297b210939a04cab8a8fd547bb34e4d549da7"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ef9d8ee2abf22c8f4d900075444e24f5a444eae3"
+        "sha": "94686b06034ff6e11217236fcaa217ec6e24bc39"
       }
     }
   ]

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:5.0.0'
+compile 'com.google.cloud:google-cloud-spanner:5.1.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "5.0.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "5.1.0"
 ```
 
 ## Authentication