fix documentation examples for with_nested

In the mailing list, Edgars Mazurs commented that the docs' example for with_nested could be wrong: https://groups.google.com/forum/#!topic/ansible-project/PpRrBQv_T9s

This made me test the examples, and it turns out the example itself was almost good: there were sql-related bugs. I've fixed them and tested the new examples before pushing. Sorry for that.