[css-view-transitions-2] Nit: use 'set' to update variable (w3c#9775)

Author: bokand

css-view-transitions-2/Overview.bs

@@ -484,7 +484,7 @@ The {{CSSViewTransitionRule}} represents a ''@view-transition'' rule.
 					[=ViewTransition/initial snapshot containing block size=] is |outboundTransition|'s [=ViewTransition/initial snapshot containing block size=],
 					and whose [=ViewTransition/is inbound cross-document transition=] is true.
 
-				1. Let |newDocument|'s [=active view transition=] be |inboundTransition|.
+				1. Set |newDocument|'s [=active view transition=] to |inboundTransition|.
 
 				1. [=Call the update callback=] for |inboundTransition|.