drm/privacy-screen: Fix sphinx warning

Fix the following warning from "make htmldocs":

drivers/gpu/drm/drm_privacy_screen.c:270:
 WARNING: Inline emphasis start-string without end-string.

Fixes: 8a12b170 ("drm/privacy-screen: Add notifier support (v2)")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Link: https://lore.kernel.org/r/20220207130407.389585-1-hdegoede@redhat.com
Link: https://patchwork.freedesktop.org/patch/msgid/20220207130407.389585-1-hdegoede@redhat.com

drivers/gpu/drm/drm_privacy_screen.c

@@ -269,7 +269,7 @@ EXPORT_SYMBOL(drm_privacy_screen_get_state);
  *
  * The notifier is called with no locks held. The new hw_state and sw_state
  * can be retrieved using the drm_privacy_screen_get_state() function.
- * A pointer to the drm_privacy_screen's struct is passed as the void *data
+ * A pointer to the drm_privacy_screen's struct is passed as the ``void *data``
  * argument of the notifier_block's notifier_call.
  *
  * The notifier will NOT be called when changes are made through