drm/doc: Polish for drm_mode_parse_command_line_for_connector

Quoting the module option format looks soo much nicer, and avoids
sphinx spewing errors about markup issues.

v2: Fix typo in commit message (Thierry).

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180219225356.24996-4-daniel.vetter@ffwll.ch