chore: regenerate README (#593)

This PR was generated using Autosynth. 🌈


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

```
2021-04-14 15:05:26,450 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-firestore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-14 15:05:27,658 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-firestore.git",
-        "sha": "32e9cb145459b8e22d6e7b6c7949040f71fc2ef7"
+        "sha": "998dcd4958c2632d673fc637139cbd12250ef7bc"
       }
     },
     {

README.md

@@ -38,7 +38,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-firestore</artifactId>
-  <version>2.2.5</version>
+  <version>2.2.6</version>
 </dependency>
 
 ```
@@ -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:2.2.5'
+compile 'com.google.cloud:google-cloud-firestore:2.2.6'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-firestore" % "2.2.5"
+libraryDependencies += "com.google.cloud" % "google-cloud-firestore" % "2.2.6"
 ```
 
 ## Authentication