[selectors-4][editorial] Removed reference to <forgiving-relative-selector-list>

The definition of `<forgiving-relative-selector-list>` got removed in w3c@4756df8 when making `:has()` unforgiving.
This change removes the last remnant reference to it.

Author: SebastianZ

selectors-4/Overview.bs

@@ -4542,7 +4542,7 @@ Grammar</h2>
 
 
 <h3 id='forgiving-selector'>
-<<forgiving-selector-list>> and <<forgiving-relative-selector-list>></h3>
+<<forgiving-selector-list>></h3>
 
 	For legacy reasons,
 	the general behavior of a selector list