change API docs sis_import_id to Integer

Test Plan:
- JSON generated files from "rake doc:api" should not have an
  "Int64" type in the sis_import_id field of the User model.
  (It should have an Integer type and an Int64 format, however)

Fixes SIS-313

Change-Id: I21fa94dab8df3499c2911ee76865be04c83e9ccb
Reviewed-on: https://gerrit.instructure.com/35918
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Eric Adams <eadams@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>
Reviewed-by: Ken Romney <kromney@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>

app/controllers/users_controller.rb

@@ -59,7 +59,8 @@
 #         "sis_import_id": {
 #           "description": "The id of the SIS import.  This field is only included if the user came from a SIS import and has permissions to manage SIS information.",
 #           "example": "18",
-#           "type": "int64"
+#           "type": "integer",
+#           "format": "int64"
 #         },
 #         "sis_login_id": {
 #           "description": "DEPRECATED: The SIS login ID associated with the user. Please use the sis_user_id or login_id. This field will be removed in a future version of the API.",