chore: regenerate README (#537)

This PR was generated using Autosynth. 🌈


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

```
2021-02-25 00:56:27,834 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-pubsub/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-02-25 00:56:29,403 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/91b253c9-3412-488e-b9bd-8e75d9d8189c/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-pubsub.git",
-        "sha": "6f172ae74512bfbcb9d2c017ff6fff6ee0478755"
+        "sha": "e304baca91dcb6f452d5220bf3db4c2b17e2fc59"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "79ab0b44a2cc7d803d07c107f9faf07729fc4012"
+        "sha": "0199c79b8324fba66476300824aa931788c47e2d"
       }
     }
   ]

README.md

@@ -17,7 +17,7 @@ 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>17.0.0</version>
+      <version>18.0.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>