Use latest version in dependencies snippet (#7545)

Please use the latest version in the snippet as the older ones may contain nasty bugs (like this one swagger-api/swagger-codegen#6122). Thanks.

Author: michael-simons

modules/swagger-codegen-maven-plugin/README.md

@@ -11,7 +11,7 @@ Add to your `build->plugins` section (default phase is `generate-sources` phase)
 <plugin>
     <groupId>io.swagger</groupId>
     <artifactId>swagger-codegen-maven-plugin</artifactId>
-    <version>2.2.3</version>
+    <version>2.3.1</version>
     <executions>
         <execution>
             <goals>