Fix typo for comments in CONTRIBUTING.md.
Fix typo for comments in CONTRIBUTING.md.

librsync/librsync,dbaarda/librsync,sourcefrog/librsync,librsync/librsync,librsync/librsync,librsync/librsync,sourcefrog/librsync,dbaarda/librsync,dbaarda/librsync,sourcefrog/librsync,dbaarda/librsync,sourcefrog/librsync