DOC: Issue #6763 - Corrected MERGE example
Corrected the example provided in the MERGE function docstring.

Closes #6763 

@jorisvandenbossche looks ok to me...merge when you are ok
 Yes, certainly OK. Thanks @SplashDance for the quick fix! Your first contribution is a fact
 Well, I'll admit -- it does feel pretty good having made my first contribution to pandas... Thanks for your help!
