chore: regenerate README (#557)

This PR was generated using Autosynth. 🌈


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

```
2021-03-03 18:33:35,908 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-pubsub/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-03 18:33:36,835 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/4dd83482-b52e-406c-ac3b-b825b81220da/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": "6b0378ee68533408d835203395c9bb4145a48608"
+        "sha": "adbcc0c6777e35eae24b538e6c48f9ef7485a786"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8c5628b86cfa8386de7b8fc1675e6b528b552d57"
+        "sha": "2e7bd5cc66a3f0b90ce1c150eb35c89eeb9872d6"
       }
     }
   ]

README.md

@@ -47,7 +47,7 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:18.0.0')
+implementation platform('com.google.cloud:libraries-bom:18.1.0')
 
 compile 'com.google.cloud:google-cloud-pubsub'
 ```