DOC: Formatting for DtypeWarning Docstring [ci skip]
xref : https://github.com/pandas-dev/pandas/pull/20208

![fireshot capture 002 - pandas errors dtypewarning pandas 0_ - file____users_taugspurger_sandbox_](https://user-images.githubusercontent.com/1312546/37349929-958f175e-26a5-11e8-8c1e-872c2261c64f.png)

cc @jorisvandenbossche @hissashirocha