drm/doc: fix reference to drm_format_modifier_blob

The documentation build system recognizes "struct XXX" references and
generates links for them.
Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201217113220.102271-4-contact@emersion.fr

drivers/gpu/drm/drm_plane.c

@@ -68,9 +68,9 @@
  *
  * IN_FORMATS:
  *     Blob property which contains the set of buffer format and modifier
- *     pairs supported by this plane. The blob is a drm_format_modifier_blob
- *     struct. Without this property the plane doesn't support buffers with
- *     modifiers. Userspace cannot change this property.
+ *     pairs supported by this plane. The blob is a struct
+ *     drm_format_modifier_blob. Without this property the plane doesn't
+ *     support buffers with modifiers. Userspace cannot change this property.
  */
 
 static unsigned int drm_num_planes(struct drm_device *dev)