Update sql_movie_rating_query_ex_challenge.md

I was going through the coursera MOOC on this course, and came across your solution on the net. Just thought I might add this. If it's wrong, let me know. The database that has been provided there, it seem to work fine for that.
Update sql_movie_rating_query_ex_challenge.md

Just thought I might add this. If anything is wrong with this query, please let me know. This query worked for me on the coursera interface.
Merge pull request #1 from alayek/patch-1

Patch 1