Fix paths in doc which are searched for matplotlibrc (XDG).

See pull request #454 (3ac81445b0ed25dd1b4b575c30c9c7cd04d66b6c)
Merge pull request #2912 from herr-biber/fix-matplotlibrc-doc-xdg-paths

Fix paths in doc which are searched for matplotlibrc (XDG).