Correct submission file upload documentation
Correct submission file upload documentation

The current documentation mentions that a file upload API is
coming soon, while it is already there for quite a while.

closes GRADE-2269
closes GH-1473

Test Plan:
  - Build documentation
  - Check if the explanation for creating a submission makes more sense now

Change-Id: I468e09fbe30b346d9433ba1425f51d92763f9fc0
Reviewed-on: https://gerrit.instructure.com/199330
Reviewed-by: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
Tested-by: Jenkins
QA-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
Product-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms