Update parent_services.rst

Clarifying which "file" we are talking about...

Author: ThomasLandauer

service_container/parent_services.rst

@@ -147,7 +147,7 @@ All attributes on the parent service are shared with the child **except** for
 
 .. note::
 
-    If you have a ``_defaults`` section in your file, all child services are required
+    If you have a ``_defaults`` section in your ``services.yaml``, all child services are required
     to explicitly override those values to avoid ambiguity. You will see a clear
     error message about this.