Trivial fix to spelling mistake in seq_printf message.

Fixes: a8b9370f ("drm/i915/guc: Dump the GuC stage descriptor pool in debugfs")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170516092235.28640-1-colin.king@canonical.com