fix bessel README

Author: masa16

gsl/bessel/README.md

@@ -13,7 +13,8 @@ def gplot(plot_args,setting,fn)
  set setting
  plot(*plot_args)
  pause mouse:"any"
- output fn+".png"
+ output fn+".png",size:[540,400]
+ #output fn+".ps","color"
  reset
  end
 end