drm/prime: fix comment on PRIME Helpers

s/Exporting/Importing

Fixes: 805dc614

 ("drm/prime: Update docs")
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723123307.31064-1-jmcasanova@igalia.com