Update ratpack.groovy file for ratpack.groovy.Groovy changes

Currently, the example app will fail to run with the following error

    ratpack.groovy: 4: unable to resolve class ratpack.groovy.RatpackScript
     @ line 4, column 1.
       import ratpack.groovy.RatpackScript
       ^

And also

    ratpack.groovy: 5: unable to resolve class ratpack.groovy.Template
     @ line 5, column 1.
       import ratpack.groovy.Template
       ^

It looks like some bits have been moving around (in a positive way). This pr ensures the sample is back in running order.