drm/vblank: Fix typo in docs

Fix typo in intro chapter in drm_vblank.c.
Change 'sacn' to 'scan'.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114142245.udr7v2aa43ho56xs@adolin