drm/doc: make drm-uapi igt-tests more readable

Correct grammar and make the use of the igt-tests more readable.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Gabriela Bittencourt <gabrielabittencourt00@gmail.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: dri-devel@lists.freedesktop.org
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20221118235137.6859-1-rdunlap@infradead.org