media: Documentation: mc-core: Fix typo

Fix a typo in the mc-core.rst media driver API documentation. Due to its
nature, the typo unfortunately caused a warning during documentation
build.

Fixes: 03b28286 ("media: mc-entity: Add a new helper function to get a remote pad for a pad")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>