chore: regenerate README (#633)

This PR was generated using Autosynth. 🌈


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

```
2021-05-14 20:24:02,291 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-firestore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-14 20:24:03,885 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/97d48566-658f-43c4-807c-4dc3917bd463/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-firestore.git",
-        "sha": "956dadcb580e59e8488bffec7ffb58bc503d8598"
+        "sha": "c68aa25ba276485a82e908a794773eaa27419b8e"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "bd8281a06cc7f84906e04d4843c1d3d386a980cd"
+        "sha": "4f4b1b9b8d8b52f1e9e4a76165896debce5ab7f1"
       }
     }
   ]

README.md

@@ -45,7 +45,7 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:20.3.0')
+implementation platform('com.google.cloud:libraries-bom:20.4.0')
 
 compile 'com.google.cloud:google-cloud-firestore'
 ```