Fix mistake in setup transition pseudo-elements algorithm

Author: Psychpsyo

css-view-transitions-1/Overview.bs

@@ -1473,7 +1473,7 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 		and generates initial styles.
 		The structure of the pseudo-tree is covered at a higher level in [[#view-transition-pseudos]].
 
-		1. Let |document| be [=this's=] [=relevant global object's=] [=associated document=].
+		1. Let |document| be |transition|'s [=relevant global object's=] [=associated document=].
 
 		1. Set |document|'s [=show view transition tree=] to true.
 

css-view-transitions-2/Overview.bs

@@ -2658,7 +2658,7 @@ It has the following [=struct/items=]:
 		and generates initial styles.
 		The structure of the pseudo-tree is covered at a higher level in [[#view-transition-pseudos]].
 
-		1. Let |document| be [=this's=] [=relevant global object's=] [=associated document=].
+		1. Let |document| be |transition|'s [=relevant global object's=] [=associated document=].
 
 		1. Set |document|'s [=show view transition tree=] to true.