Update the link to colormaps documentation

plotnine/scales/scale_color.py

@@ -455,7 +455,7 @@ class scale_color_cmap(scale_continuous):
         A standard Matplotlib colormap name. The default is
         `viridis`. For the list of names checkout the output
         of `matplotlib.colormaps()` or see the
-        [documentation](http://matplotlib.org/users/colormaps.html).
+        [documentation](https://matplotlib.org/stable/users/explain/colors/colormaps.html).
     {superclass_parameters}
     na_value : str, default="#7F7F7F"
         Color of missing values.