chore: regenerate README (#231)

This PR was generated using Autosynth. 🌈


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

```
2020-10-21 18:37:17,688 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-document-ai/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-10-21 18:37:18,644 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/731fd9d5-2619-4b18-82f1-f135764623ab/targets

- [ ] To automatically regenerate this PR, check this box.

Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-document-ai.git",
-        "sha": "3df492b192aef2dc21d4ce37e23a6b5a21a55483"
+        "sha": "f0afd0888822e4e9d3e91d9514ba455e910fc5ee"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "6abb59097be84599a1d6091fe534a49e5c5cf948"
+        "sha": "901ddd44e9ef7887ee681b9183bbdea99437fdcc"
       }
     }
   ]

README.md

@@ -20,7 +20,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>13.0.0</version>
+      <version>13.1.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>