chore: regenerate README (#570)

This PR was generated using Autosynth. 🌈


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

```
2021-05-31 22:38:23,212 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-monitoring/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-31 22:38:24,426 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/333a93c0-36bd-4142-a8cf-37d4984bfb8d/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-monitoring.git",
-        "sha": "f9ba30d2e1da96953333283b2ef284a1d4676859"
+        "sha": "983729b6af0b72e103a402b96eb06d486fb7e6d7"
       }
     },
     {

README.md

@@ -39,7 +39,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-monitoring</artifactId>
-  <version>2.3.0</version>
+  <version>2.3.1-SNAPSHOT</version>
 </dependency>
 
 ```