DOC: Fix typo in docstring.
DOC: Fix typo in docstring.

Scapogo/zipline,Scapogo/zipline,magne-max/zipline-ja,bartosh/zipline,bartosh/zipline,alphaBenj/zipline,florentchandelier/zipline,enigmampc/catalyst,humdings/zipline,quantopian/zipline,alphaBenj/zipline,humdings/zipline,quantopian/zipline,magne-max/zipline-ja,enigmampc/catalyst,florentchandelier/zipline