chore: regenerate README (#905)

This PR was generated using Autosynth. 🌈


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

```
2021-02-23 22:33:45,447 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-02-23 22:33:47,224 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/6af050d2-f043-4217-a10e-c52d9cc501b8/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": "7d4f70a613b9a7668e371ee3aa174fdf897de141"
+        "sha": "2e0ebed3c7e8967245cf7c857fc7f9232ce9bb93"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "4dca4132c6d63788c6675e1b1e11e7b9225f8694"
+        "sha": "79ab0b44a2cc7d803d07c107f9faf07729fc4012"
       }
     }
   ]

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:4.0.0'
+compile 'com.google.cloud:google-cloud-spanner:4.0.1'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "4.0.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "4.0.1"
 ```
 
 ## Authentication