Remove a one-too-many dash from the parallel switch

`---parallel` contains a dash too much, which results in `no such option: ---parallel`.

Author: MWeesenaar

Makefile

@@ -24,7 +24,7 @@ migrations:
 coverage:
 	coverage erase
 	DJANGO_SETTINGS_MODULE=tests.settings PYTHONPATH=. \
-		coverage run ---parallel --source=two_factor \
+		coverage run --parallel --source=two_factor \
 		`which django-admin` test ${TARGET}
 	coverage combine
 	coverage html