chore: regenerate README (#590)

This PR was generated using Autosynth. 🌈


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

```
2021-04-13 20:08:12,228 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-firestore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-13 20:08:13,516 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/5b5aaa34-f641-4e4e-9cce-0bdf81fd0350/targets

- [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-firestore.git",
-        "sha": "029f01d808afbeb531ee70b78f72bccaadfeb1e0"
+        "sha": "32e9cb145459b8e22d6e7b6c7949040f71fc2ef7"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "0a071b3460344886297a304253bf924aa68ddb7e"
+        "sha": "082e1ca0863b13ada8594fe91845380765da5b70"
       }
     }
   ]

README.md

@@ -17,7 +17,7 @@ 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>19.0.0</version>
+      <version>20.0.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>