doc: releases: Fix GH link in migration guide v3.6

Putting a space between :github: and the PR number causes the rendered
text to not link to the issue.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
(cherry picked from commit 5a49b7c90d8780e2a145b4cddab04f35b1d6c21a)
1 file changed