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>

Documentation/userspace-api/media/v4l/libv4l-introduction.rst

@@ -136,9 +136,9 @@ V4L2 functions
 
    operates like the :c:func:`read()` function.
 
-.. c:function:: void v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset);
+.. c:function:: void *v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset);
 
-   operates like the :c:func:`munmap()` function.
+   operates like the :c:func:`mmap()` function.
 
 .. c:function:: int v4l2_munmap(void *_start, size_t length);