fixes typo in plot docstring
fixes typo in plot docstring

bretthandrews/marvin,bretthandrews/marvin,sdss/marvin,albireox/marvin,albireox/marvin,sdss/marvin,sdss/marvin,bretthandrews/marvin,albireox/marvin,albireox/marvin,bretthandrews/marvin,sdss/marvin