chore: regenerate README (#317)

This PR was generated using Autosynth. 🌈


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

```
2021-01-28 17:32:48,626 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-logging-logback/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-28 17:32:49,535 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/f80c1ff4-5d53-4757-bbe5-e37b778b60b3/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-logging-logback.git",
-        "sha": "608a7c2b7e71312a2f3907539dcf32cb42f92948"
+        "sha": "dddbeaf35f6825bded644b4abff9da39cd2bdda2"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "3816b080296d4d52975079fd26c110dd26ba25af"
+        "sha": "57c23fa5705499a4181095ced81f0ee0933b64f6"
       }
     }
   ]

README.md

@@ -177,7 +177,7 @@ Java 11 | [![Kokoro CI][kokoro-badge-image-5]][kokoro-badge-link-5]
 
 Java is a registered trademark of Oracle and/or its affiliates.
 
-[product-docs]: https://cloud.google.com/logging/docs/setup/java#logback_appender_for
+[product-docs]: https://cloud.google.com/logging-logback/docs/
 [javadocs]: https://googleapis.dev/java/google-cloud-logging-logback/latest/index.html
 [kokoro-badge-image-1]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-logging-logback/java7.svg
 [kokoro-badge-link-1]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-logging-logback/java7.html