docs: fix 18.0 release note version

Fixes: 839fb3a6 "docs: Update 18.0.0 release notes"
Cc: "18.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit d77844a5)