chore: regenerate README (#594)

This PR was generated using Autosynth. 🌈


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

```
2021-09-02 01:21:44,227 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-09-02 01:21:46,100 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/56eab260-2a0e-4455-80ae-e9acfadb443b/targets

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Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner-jdbc.git",
-        "sha": "7b341394dfe0ee7cae5131681ee72d9750eccce1"
+        "sha": "5e1504d217563e4b9cd34211cbca55856fad0e60"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "dfd55ad78a700acf987d592c8279789b1319b8c5"
+        "sha": "5c0fa62eea9c33ebe61e582424b659eb264e1ba4"
       }
     }
   ]

README.md

@@ -19,20 +19,20 @@ 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>2.4.0</version>
+  <version>2.4.1</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 
 ```Groovy
-implementation 'com.google.cloud:google-cloud-spanner-jdbc:2.4.0'
+implementation 'com.google.cloud:google-cloud-spanner-jdbc:2.4.1'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "2.4.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "2.4.1"
 ```
 
 ## Authentication