drm: ci: docs: fix build warning - add missing escape

Fix the following warning:

Documentation/gpu/automated_testing.rst:55: WARNING: Inline emphasis start-string without end-string.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230824164230.48470-1-helen.koike@collabora.com