Fix the top_artists example

Hi, I've noticed two types of errors when running top_artists:

* The open() function wasn't working as expected with `TypeError: coercing to Unicode: need string or buffer, File found`. Is this my python version? I'm not sure... but my commit fixes it to behave similar to other places
* Line 27 was missing a few spaces as a result of probably previous bogus commit (I guess 11eb06b35e10ceaecb584234643b14ce2dfbc59d)