chore: regenerate README (#450)

This PR was generated using Autosynth. 🌈


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

```
2021-05-10 17:34:18,659 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-iot/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-10 17:34:19,871 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

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https://source.cloud.google.com/results/invocations/e29bc227-38e4-4781-a41d-b581ca76b307/targets

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Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-iot.git",
-        "sha": "6c39e84e6434e565cc78dc2f0957a04330e8bacc"
+        "sha": "af147c95015d82f97aa2006a45f48bd06d07f188"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "06a8cd0ff7e81b05e6c503eab510ec622384caa7"
+        "sha": "6726988c677bb78385868bfc48dbfa2fe981d44a"
       }
     }
   ]

README.md

@@ -45,7 +45,7 @@ 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:20.2.0')
+implementation platform('com.google.cloud:libraries-bom:20.3.0')
 
 compile 'com.google.cloud:google-cloud-iot'
 ```