doc: contribute: fix dead link in RFC proposal template

Fix a broken link in the 'contribute/proposals_and_rfcs.rst' file that
pointed to a non-existent Markdown RFC template.

The link previously pointed to:
  .github/ISSUE_TEMPLATE/003_rfc-proposal.md

It has been corrected to point to the actual YAML file:
  .github/ISSUE_TEMPLATE/003_rfc-proposal.yml

Fixes: #92342

Signed-off-by: Gaetan Perrot <gaetan.perrot@spacecubics.com>