update javaGuide3.md - correct onApplicationStart method name to onStart

This Patch corrects the GlobalSettings method name as described:

In JavaGuide3.md - section Bootstrapping with default data:
The paragraph called out onApplicationStart() as the method name in GlobalSettings.  

However, the API for GlobalSettings defines the method as onStart().
http://www.playframework.com/documentation/api/2.1.x/java/index.html
Merge pull request #1392 from richardbroersma/patch-2

update javaGuide3.md - correct onApplicationStart method name to onStart