[REFACTOR] Fix copywriting

apps/accounts/views.py

@@ -61,7 +61,7 @@ class AccountViewSet(viewsets.ModelViewSet):
 
         if User.objects.filter(username=username).exists():
             return Response(
-                {"username" : ["User with that username already exists"]},
+                {"username" : ["User with that username already exists."]},
                 status=status.HTTP_409_CONFLICT
             )