powerlaw_xyfit.sl: Fix typo in help

src/fitting/xyfit/fit-functions/powerlaw_xyfit.sl

@@ -8,7 +8,7 @@ define powerlaw_xyfit()
 %\description
 %    This function is not meant to be called directly!
 %    
-%    Calling \code{xyfit_fun ("powerlawlinear");} sets up a powerlaw fit
+%    Calling \code{xyfit_fun ("powerlaw");} sets up a powerlaw fit
 %    function for xy-data. It has the form \code{y = norm*x^{-index}}
 %\seealso{xyfit_fun, define_xydata, plot_xyfit, linear_regression}
 %!%-