Update testing.rst

Co-authored-by: Oskar Stark <oskarstark@googlemail.com>

Author: alexislefebvre

testing.rst

@@ -592,7 +592,7 @@ container is a service that can be get via the normal container::
 
 .. tip::
 
-    The special container `test.service_container` doesn't give access
+    The special container ``test.service_container`` doesn't give access
     to the services that have been removed. In this case, you'll have
     to declare that service as public.