chore: regenerate README (#859)

This PR was generated using Autosynth. 🌈


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

```
2021-06-04 14:52:35,890 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigtable/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-04 14:52:37,164 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/943495b6-e956-4fbb-9062-0fe29cce4f34/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": "1d8bddd4f2a9eef8628d198e6b3beafc920643ca"
+        "sha": "e7773eed9f34ab4d104df5c09aefb5a302ab4186"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "a2de127f90799cd5bace653b1dcc5dd2ca8b19c9"
+        "sha": "8da7927493739fa36130360019ca21258242f73a"
       }
     }
   ]

README.md

@@ -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:1.25.0'
+compile 'com.google.cloud:google-cloud-bigtable:1.26.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "1.25.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "1.26.0"
 ```
 
 ## Authentication