service name should match the instructions in wiki

need to s/mongodb-hello-mongo/mongodb-hello/ in order to be consistent with the getting started instructions and screenshots (https://github.com/SpringSource/cloudfoundry-samples/wiki/Spring-Hello-MongoDB-Sample-Application)

Currently, since service name is not updated, the application throws error when trying to start
Error: Application 'samples-spring's state is undetermined, not enough information available.

The logs complain about null argument being passed to constructor of the class org.springframework.data.document.mongodb.MongoTemplate