docs: fix URL in CONTRIBUTING.rst
docs: fix URL in CONTRIBUTING.rst

Remove the unnecessary '/start/' from the link to the CONTRIBUTING
documentation.

This will ensure that users that would like to contribute to Kolibri are
directed to the right place and don't encounter a 404.

learningequality/kolibri,indirectlylit/kolibri,mrpau/kolibri,lyw07/kolibri,indirectlylit/kolibri,lyw07/kolibri,learningequality/kolibri,mrpau/kolibri,mrpau/kolibri,mrpau/kolibri,indirectlylit/kolibri,lyw07/kolibri,learningequality/kolibri,lyw07/kolibri,indirectlylit/kolibri,learningequality/kolibri