[css-pseudo-4] Fix remaining reference to old part-like pseudo-element term.

Author: dbaron

css-pseudo-4/Overview.bs

@@ -1535,7 +1535,7 @@ List Markers: the ''::marker'' pseudo-element</h3>
   ISSUE: Interaction of ''::marker'' and ''::first-line'' is currently under discussion in <a href='https://github.com/w3c/csswg-drafts/issues/4506'>Issue 4506</a>.
 
   ''::marker'' is a [=tree-abiding pseudo-element=],
-  but not [=part-like pseudo-element|part-like=];
+  but is not [=element-backed=];
   only a limited set of properties can be used on the ''::marker'' pseudo-element.
   This list is defined in [[css-lists-3#marker-properties]].
 
@@ -2053,4 +2053,4 @@ Acknowledgements</h2>
     get-computed-style-crash.html
     svg-text-selection-002.html
     text-selection.html
-  </wpt>
+  </wpt>