V4L/DVB: v4l doc: fix font of field name

The field 'reserved' was not tagged as a 'structfield' as it should.
This made the text a bit confusing.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/v4l/vidioc-reqbufs.xml

@@ -61,7 +61,7 @@ fields of the <structname>v4l2_requestbuffers</structname> structure.
 They set the <structfield>type</structfield> field to the respective
 stream or buffer type, the <structfield>count</structfield> field to
 the desired number of buffers, <structfield>memory</structfield>
-must be set to the requested I/O method and the reserved array
+must be set to the requested I/O method and the <structfield>reserved</structfield> array
 must be zeroed. When the ioctl
 is called with a pointer to this structure the driver will attempt to allocate
 the requested number of buffers and it stores the actual number