chore: regenerate README (#639)

This PR was generated using Autosynth. 🌈


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

```
2021-05-17 22:19:02,923 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-firestore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-17 22:19:04,196 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/baaba5f0-e108-40b9-94eb-ba48c1326680/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": "2bff9d2b227b62ee9550cbd6d625d9424d314a9f"
+        "sha": "25fc44fa14df81ddb288431852d2db6f9ab92455"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "4f4b1b9b8d8b52f1e9e4a76165896debce5ab7f1"
+        "sha": "c86c7a60985644eab557949363a38301d40d78d2"
       }
     }
   ]

README.md

@@ -51,12 +51,12 @@ compile 'com.google.cloud:google-cloud-firestore'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-firestore:2.3.0'
+compile 'com.google.cloud:google-cloud-firestore:2.4.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-firestore" % "2.3.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-firestore" % "2.4.0"
 ```
 
 ## Authentication