Corrected local execution cmd-line (GoogleCloudPlatform#2282)

Looks like bad copypasta. The updated cmd-line works correctly for me.

Author: kintel

appengine-java11/helloworld-servlet/README.md

@@ -85,7 +85,7 @@ mvn install
 
 - Start the server with your `WAR` file as an argument:
 ```
-mvn exec:java -Dexec.args="../helloworld-java8/target/helloworld.war"
+mvn exec:java -Dexec.args="../helloworld-servlet/target/helloworld.war"
 ```
 
 Then visit: http://localhost:8080/hello