chore: regenerate README (#852)

This PR was generated using Autosynth. 🌈


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

```
2021-08-16 22:19:20,818 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-asset/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-16 22:19:22,141 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/d6f14c3d-ca3f-410c-a273-dc12bb3eb23e/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-asset.git",
-        "sha": "84e9747db1ea647a7432751e4c1c20a14484c984"
+        "sha": "df575315598d72ae1bf26e49b80e7e3755392c41"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "7e1f6da50524b5d98eb67adbf6dd0805df54233d"
+        "sha": "a01e1a0bf70754f51450958b966bd673945d1e6e"
       }
     }
   ]

README.md

@@ -50,7 +50,7 @@ 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:20.9.0')
+implementation platform('com.google.cloud:libraries-bom:21.0.0')
 
 compile 'com.google.cloud:google-cloud-asset'
 ```