media: docs: v4l2grab.c.rst: change unintended assignment

Example should be comparing errno to EINTR instead of doing assignment.

Signed-off-by: Daniel Lundberg Pedersen <dlp@qtec.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>