fix returns in `pvsystem.sapm` docstring (#234)
fix returns in `pvsystem.sapm` docstring (#234)

* `Imp` should be lower case `i_mp`
anomam/pvlib-python,cwhanse/pvlib-python,rubennj/pvlib-python,pvlib/pvlib-python,mikofski/pvlib-python,alorenzo175/pvlib-python,wholmgren/pvlib-python,uvchik/pvlib-python