chore: regenerate README (#901)

This PR was generated using Autosynth. 🌈


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

```
2021-07-08 17:54:35,144 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigtable/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-08 17:54:36,613 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/7b3cfc9b-a7f7-4a63-a8dd-4714bc7601d7/targets

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Author: yoshi-automation

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigtable.git",
-        "sha": "5d036b2a2525fe3b2a0f1cbe0f92f3e5e63876b9"
+        "sha": "fba970daf057afe8a7294194b8529d90843634e8"
       }
     },
     {

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:20.7.0')
+implementation platform('com.google.cloud:libraries-bom:20.8.0')
 
 compile 'com.google.cloud:google-cloud-bigtable'
 ```