media: Documentation: v4l: Fix language

Remove extra "is" from sentence explaining V4L2_CID_PIXEL_RATE may be used
to tell the pixel rate used by a transmitter driver. Also call
V4L2_CID_PIXEL_RATE a control.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>