chore: regenerate README (#563)

This PR was generated using Autosynth. 🌈


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

```
2021-08-09 17:56:54,806 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-cloudbuild/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-09 17:56:56,639 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/4adf00ea-7076-429e-9754-a8a36da38ae5/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-cloudbuild.git",
-        "sha": "d3ebcc82a5654d33a621119d73cdf55de80cf479"
+        "sha": "fedb5cfbcc71383999a739cb41ca7a2ad86170a1"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "3d32990b6f4b57be77455e3ca89e6e21cd0c06b3"
+        "sha": "5605f38f153e7627ba4441a2c5181060b664d224"
       }
     }
   ]

README.md

@@ -56,13 +56,13 @@ compile 'com.google.cloud:google-cloud-build'
 If you are using Gradle without BOM, add this to your dependencies
 
 ```Groovy
-compile 'com.google.cloud:google-cloud-build:2.6.3'
+compile 'com.google.cloud:google-cloud-build:2.6.4'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-build" % "2.6.3"
+libraryDependencies += "com.google.cloud" % "google-cloud-build" % "2.6.4"
 ```
 
 ## Authentication