fix 'assignment_visibility' API doc type
fix 'assignment_visibility' API doc type

Swagger fails to generate code for pandarus / live API docs
because assignment_visibility array does not specify a type.

This sets the type to 'integer'.

Fixes SIS-400

Change-Id: I0a675dc51907d1a64993a36bed94d022deae3dda
Reviewed-on: https://gerrit.instructure.com/38575
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Ken Romney <4369dabf8e7d5ba636f3ef79948e6ca3ce23a99f@instructure.com>
Product-Review: Duane Johnson <0a02221ea64f4c9685145b5f44cf84715b4a8bb5@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>

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