More precise wording for "Calculate depth for node"

As discussed in w3c#3840

Author: Aleksandar Totic

resize-observer-1/Overview.bs

@@ -472,7 +472,7 @@ To <dfn>deliver resize loop error notification</dfn> run these steps:
 
 To <dfn>calculate depth for node</dfn>, given a |node|, run these steps:
 
-    1. Let |p| be the parent-traversal path from |node| to a root Element of this element's DOM tree.
+    1. Let |p| be the parent-traversal path from |node| to a root Element of this element's flattened DOM tree.
 
     2. Return number of nodes in |p|.