fixed documentation bug as per issue #4029

Docstring for iterables/rotate_right read "Left rotates" instead of "Right rotates". Mentioned in [issue #4029](https://code.google.com/p/sympy/issues/detail?id=4029&sort=-opened&colspec=ID%20Type%20Status%20Priority%20Milestone%20Reporter%20Summary%20Stars%20Opened) in the Google bug tracker.