Fixed a RST reference

Author: javiereguiluz

service_container/3.3-di-changes.rst

@@ -408,7 +408,7 @@ In general, the new best practice is to use normal constructor dependency inject
 (or "action" injection in controllers) instead of fetching public services via
 ``$this->get()`` (though that does still work).
 
-_service_autoconfigure:
+.. _service_autoconfigure:
 
 4) Auto-tagging with autoconfigure
 ----------------------------------