media: buffer.rst: fix incorrect :c:type

:c:type:`struct v4l2_requestbuffers` should be
struct :c:type:`v4l2_requestbuffers`.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>