update gradle example

[stianrincon]

As we updated the documentation about gradle, we should update the gradle examples, these ones:

https://github.com/mapstruct/mapstruct-examples/tree/master/mapstruct-on-gradle

[filiphr]

Thanks for creating the issue @stianrincon

[filiphr]

I pressed the button too fast. Thanks for creating the issue. I have moved it in the examples repository. I will review and merge the PR asap