Added example of assertNumQueries() with "using" kwarg in testing topics docs.

docs/topics/testing/tools.txt

@@ -1675,9 +1675,12 @@ your test suite.
     ``num`` database queries are executed.
 
     If a ``"using"`` key is present in ``kwargs`` it is used as the database
-    alias for which to check the number of queries.  If you wish to call a
-    function with a ``using`` parameter you can do it by wrapping the call with
-    a ``lambda`` to add an extra parameter::
+    alias for which to check the number of queries::
+
+        self.assertNumQueries(7, using='non_default_db')
+
+    If you wish to call a function with a ``using`` parameter you can do it by
+    wrapping the call with a ``lambda`` to add an extra parameter::
 
         self.assertNumQueries(7, lambda: my_function(using=7))