chore: regenerate README (#262)

This PR was generated using Autosynth. 🌈


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

```
2020-11-05 23:07:53,809 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-document-ai/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-11-05 23:07:54,712 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/6675474a-0f79-4ded-9330-342357f399b5/targets

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

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-document-ai.git",
-        "sha": "cb33a6ef64f01e2dc24ec4db04fa032bc23617a7"
+        "sha": "7e948855a403d345807f2628768f3950dbc5d8b4"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ba9918cd22874245b55734f57470c719b577e591"
+        "sha": "2c8aecedd55b0480fb4e123b6e07fa5b12953862"
       }
     }
   ]

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>14.4.1</version>
+      <version>15.0.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>