chore: regenerate README (#509)

This PR was generated using Autosynth. 🌈


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

```
2021-07-27 23:16:43,788 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-texttospeech/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-27 23:16:44,946 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/502bebd5-52a9-483b-946a-375f4cbee868/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-texttospeech.git",
-        "sha": "e38925d4a9eeb5285a6d19d2919a502f798d8def"
+        "sha": "2f31332e3bbaa2db39d25ef8969eb14fb57fe984"
       }
     },
     {

README.md

@@ -19,7 +19,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>20.8.0</version>
+      <version>20.9.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>