chore: regenerate README (#475)

This PR was generated using Autosynth. 🌈


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

```
2021-07-27 23:38:50,459 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-billing/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-27 23:38:51,644 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

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https://source.cloud.google.com/results/invocations/d1290adb-c75c-43f4-9728-6fc97fa96925/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-billing.git",
-        "sha": "e2d00100bbbda3882009ed8be6a13c5a25e8689c"
+        "sha": "473f9cf9464c26556396e187f962a3115b407a85"
       }
     },
     {