chore: regenerate README (#384)

This PR was generated using Autosynth. 🌈


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

```
2021-08-19 23:23:24,790 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-notification/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-19 23:23:26,120 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/c0631443-5765-4b7e-aacd-da4cbc8d729e/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-notification.git",
-        "sha": "5fffd8b20c5227c2cd63c109d87ccf2afa2d24f5"
+        "sha": "a101519c63928e178b6627714ae4f8b98a62f6c2"
       }
     },
     {

README.md

@@ -22,20 +22,20 @@ If you are using Maven, add this to your pom.xml file:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-notification</artifactId>
-  <version>0.122.5-beta</version>
+  <version>0.122.6-beta</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 
 ```Groovy
-compile 'com.google.cloud:google-cloud-notification:0.122.5-beta'
+compile 'com.google.cloud:google-cloud-notification:0.122.6-beta'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-notification" % "0.122.5-beta"
+libraryDependencies += "com.google.cloud" % "google-cloud-notification" % "0.122.6-beta"
 ```
 
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