add missing assignment_id param to bulk grade docs
add missing assignment_id param to bulk grade docs

Change-Id: I048e17d00f60df10e13e78264f28bb343592a77b
Reviewed-on: https://gerrit.instructure.com/108407
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Tested-by: Jenkins
Product-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
QA-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>

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