CONTRIBUTING.md: Revise text RE: target branch for docs changes
CONTRIBUTING.md: Revise text RE: target branch for docs changes

Based on feedback in #2398.

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