rust/drm/gem: Fix missing header in `Object` rustdoc

rust/drm/gem: Fix missing header in `Object` rustdoc

commit e54ad0cd3673c93cdafda58505eaa81610fe3aef upstream.

Invariants should be prefixed with a # to turn it into a header.

There are no functional changes in this patch.

Cc: stable@vger.kernel.org
Fixes: c284d3e42338 ("rust: drm: gem: Add GEM object abstraction")
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20251107202603.465932-1-lyude@redhat.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>