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