fix propTypes
fix propTypes

test plan: the following console warning doesn't appear:

Warning: Component K5AddAnnouncementButton declared `PropTypes` instead of `propTypes`. Did you misspell the property assignment?

Change-Id: If4fa93439efd0c63777e7bdc06ea652f1976d7f5
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/271908
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
QA-Review: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

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