chore: regenerate README (#727)

This PR was generated using Autosynth. 🌈


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

```
2021-08-10 22:27:37,501 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-firestore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-10 22:27:38,922 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/9b4e9b8b-3a0f-4d59-ba41-e42e136c6aaa/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": "7d7d7166716d02b5b118c1ba67dc1d52ddd3ea73"
+        "sha": "2c7f0d845d0f44dbaf81b44d2cae1a87a2e55ccf"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "5605f38f153e7627ba4441a2c5181060b664d224"
+        "sha": "6252f2cd074c38f37b44abe5e96d128733eb1b61"
       }
     }
   ]

README.md

@@ -41,7 +41,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-firestore</artifactId>
-  <version>2.6.2</version>
+  <version>3.0.0</version>
 </dependency>
 
 ```
@@ -56,13 +56,13 @@ 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:2.6.2'
+compile 'com.google.cloud:google-cloud-firestore:3.0.0'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-firestore" % "2.6.2"
+libraryDependencies += "com.google.cloud" % "google-cloud-firestore" % "3.0.0"
 ```
 
 ## Authentication