chore: regenerate README (#920)

This PR was generated using Autosynth. 🌈


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

```
2021-07-20 14:56:38,808 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigtable/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-20 14:56:40,835 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/472c878d-7d9b-4576-aa83-a515f3fab98f/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-bigtable.git",
-        "sha": "747c56a43e8487f4e6d823294dfb75f727b7fdda"
+        "sha": "68d80f3a6182714d746f9c5b2ae384f364525dd9"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "4124d245133c0f740ac5e6fa2e7daeba327f1297"
+        "sha": "52e4e46eff2a0b70e3ff5506a02929d089d077d4"
       }
     }
   ]

README.md

@@ -38,7 +38,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-bigtable</artifactId>
-  <version>1.27.2</version>
+  <version>1.27.3</version>
 </dependency>
 
 ```
@@ -51,12 +51,12 @@ compile 'com.google.cloud:google-cloud-bigtable'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-bigtable:1.27.2'
+compile 'com.google.cloud:google-cloud-bigtable:1.27.3'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "1.27.2"
+libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "1.27.3"
 ```
 
 ## Authentication