api: correctly document course-level participation data endpoint
api: correctly document course-level participation data endpoint

fixes CNVS-18048

test plan:
- hit /api/v1/courses/X/analytics/activity
- the result should match the documentation

Change-Id: I0069cae3d8953c47950f012a50f79e59b001a41b
Reviewed-on: https://gerrit.instructure.com/101818
Tested-by: Jenkins
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

SwinburneOnline/canvas-analytics,instructure/analytics,instructure/analytics,instructure/analytics,SwinburneOnline/canvas-analytics,SwinburneOnline/canvas-analytics