Fix typo in view-transitions-1 (#12766)

Author: Psychpsyo

css-view-transitions-1/Overview.bs

@@ -1162,7 +1162,7 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 		:: A boolean. Initially false.
 
 			When this is true, [=this=]'s [=active view transition=]'s [=ViewTransition/transition root pseudo-element=] renders as a child of [=this=]'s [=document element=],
-			with [=this=]'s [=document element=] is its [=originating element=].
+			with [=this=]'s [=document element=] being its [=originating element=].
 
 			Note: The position of the [=ViewTransition/transition root pseudo-element=] within the [=document element=] does not matter, as the [=ViewTransition/transition root pseudo-element=]'s [=containing block=] is the [=snapshot containing block=].