chore: regenerate README (#430)

This PR was generated using Autosynth. 🌈


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

```
2021-04-19 20:38:16,231 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-document-ai/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-19 20:38:17,520 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-document-ai.git",
-        "sha": "4accfd0f9a5af72b51e6436503959c8f692b26f2"
+        "sha": "1ea5c6128ff9177a9cd21705adaffe6e095476fb"
       }
     },
     {

README.md

@@ -55,12 +55,12 @@ compile 'com.google.cloud:google-cloud-document-ai'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-document-ai:0.6.0'
+compile 'com.google.cloud:google-cloud-document-ai:0.7.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-document-ai" % "0.6.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-document-ai" % "0.7.0"
 ```
 
 ## Authentication