media: v4l2-mem2mem.h: fix typo in comment

The comment describing the function v4l2_m2m_last_src_buf() says that this
function returns the last destination buffer when it actually returns the
last source buffer. Fix the comment so that it is accurate.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>