CONTRIBUTING.md: Remove non-wroking links
CONTRIBUTING.md: Remove non-wroking links

Because the three types of contributions are now an enumerated list rather than headings (42bc1215), they no longer spawn anchors for links.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3