DOC: typo fix

This just fixes a minor typo.
Merge pull request #4399 from certik/docs

DOC: typo fix
Merge pull request #4388 from jreback/GH4377

BUG: fixes for GH4377
BUG: Fix .iat indexing with a PeriodIndex GH4390
     Series.get with negative indexers returns the same as []