Directly link matplotlib.org and not sourceforge.net

Due to sourceforge's recent practice of adding AdWare to some downloads, it gets blocked if you use the ublock browser extension. Since the linked sourceforge page redirects to matplotlib.org anyway, it doesn't seem to make sense to not directly link to matplotlib.org.

Author: yaph

doc/users/style_sheets.rst

@@ -84,6 +84,6 @@ changes, you can write something like the following::
    >>> plt.show()
 
 
-.. _matplotlibrc: http://matplotlib.sourceforge.net/users/customizing.html
+.. _matplotlibrc: http://matplotlib.org/users/customizing.html
 .. _ggplot: http://had.co.nz/ggplot/
 .. _R: http://www.r-project.org/