change 'muted' attr of Assignment to boolean

Assignment object is documented as having a "muted" attribute of
Integer type. It should be "Boolean".

Fixes SIS-309

Change-Id: Ibad9ff20bae0f68fa642187fc1a250f869e80633
Reviewed-on: https://gerrit.instructure.com/35816
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>