Minor fix to docstring.
Minor fix to docstring.

kyleabeauchamp/pymbar,jchodera/pymbar,jchodera/pymbar,mrshirts/pymbar,wesbarnett/pymbar,mrshirts/pymbar,jchodera/pymbar,mrshirts/pymbar,kyleabeauchamp/pymbar,wesbarnett/pymbar,wesbarnett/pymbar,Lnaden/pymbar,kyleabeauchamp/pymbar,jchodera/pymbar,mrshirts/pymbar,choderalab/pymbar,Lnaden/pymbar,choderalab/pymbar,Lnaden/pymbar,wesbarnett/pymbar,kyleabeauchamp/pymbar,choderalab/pymbar