chore: regenerate README (googleapis#531)

This PR was generated using Autosynth. 🌈


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

```
2021-07-07 14:59:54,920 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-07 14:59:56,130 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/ba544d15-79a9-4161-b4d1-03f316693e40/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner-jdbc.git",
-        "sha": "37023b8295c97304aa0a55a28f71905fcbf5b93f"
+        "sha": "0c3d02f41a4a5c5d3c3100c247a3ea2adc9090ff"
       }
     },
     {

README.md

@@ -17,18 +17,18 @@ If you are using Maven, add this to your pom.xml file:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner-jdbc</artifactId>
-  <version>2.2.5</version>
+  <version>2.2.6</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner-jdbc:2.2.5'
+compile 'com.google.cloud:google-cloud-spanner-jdbc:2.2.6'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "2.2.5"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "2.2.6"
 ```
 
 ## Authentication