doc: project: Fix collaborator reference

Commit 52e8b105 (doc: project: clarify
WG member eligibility) broke the link to the collaborator section by
using nested inline markup, which is currently not supported:

https://docutils.sourceforge.io/FAQ.html#is-nested-inline-markup-possible



As the various workarounds are not recommended by Docutils, this change
simply ends the bold formatting before the link.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>