chore: regenerate README (#223)

This PR was generated using Autosynth. 🌈


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

```
2021-01-07 21:33:38,412 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-notification/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-07 21:33:39,286 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/8776903b-8d28-45d9-a396-d94622e81adc/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-notification.git",
-        "sha": "961ae0b8850eb3e4a51fb5d35d90391020d79b95"
+        "sha": "6005f699efb017e88c6347e90e981169f1bf5318"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "41e998d5afdc2c2143a23c9b044b9931936f7318"
+        "sha": "41a4e56982620d3edcf110d76f4fcdfdec471ac8"
       }
     }
   ]

README.md

@@ -24,10 +24,11 @@ If you are using Maven, add this to your pom.xml file:
 </dependency>
 ```
 
-If you are using Gradle, add this to your dependencies
+If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
 compile 'com.google.cloud:google-cloud-notification:0.121.5-beta'
 ```
+
 If you are using SBT, add this to your dependencies
 ```Scala
 libraryDependencies += "com.google.cloud" % "google-cloud-notification" % "0.121.5-beta"