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

wilsonkichoi/zipline,grundgruen/zipline,Scapogo/zipline,alphaBenj/zipline,florentchandelier/zipline,humdings/zipline,bartosh/zipline,nborggren/zipline,bartosh/zipline,alphaBenj/zipline,grundgruen/zipline,umuzungu/zipline,florentchandelier/zipline,wilsonkichoi/zipline,magne-max/zipline-ja,nborggren/zipline,umuzungu/zipline,magne-max/zipline-ja,quantopian/zipline,Scapogo/zipline,enigmampc/catalyst,enigmampc/catalyst,quantopian/zipline,humdings/zipline