DOC: Add example to pandas.DataFrame.applymap
Examples are great - how would you feel about having one that distinguishes `applymap`? `df.apply(abs)` gives the same result there
 Further, `abs` is actually not a really good example as a functions for which you need `applymap`, as `abs(df)` will also work due to delegation to `__abs__`
 I've updated this to use a lambda function that operates element-wise.
 :+1: 
 Thanks!
