correct erroneous curl example in API documentation

fixes CNVS-12214

test plan:
  - ensure the API documentation for GroupsController#invite shows
    each email address in a separate -F argument in the curl example

Change-Id: I14691cda60c762e280b3a30d7bdbf6ef5a80b6a3
Reviewed-on: https://gerrit.instructure.com/39721
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>