media: docs: libv4l-introduction.rst: Fix function signature and link

v4l2_mmap returns a void*, also link to mmap instead of munmap

Signed-off-by: Daniel Lundberg Pedersen <dlp@qtec.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>