Update css-shared-element-transitions-1/Overview.bs

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

Author: vmpstr

css-shared-element-transitions-1/Overview.bs

@@ -169,7 +169,7 @@ the following style in the [=user-agent origin=].
 
 <div class=note>This property causes the user-agent to both capture separate
 snapshots from the elements, as well as create separate pseudo-element
-structure representing this element's "before" and "after" states. Note that
+sub-trees representing this element's "before" and "after" states. Note that
 for the purposes of this API, if one element has a tag "foo" in the before
 state, and another element has a tag "foo" in the after state, they are treated
 as representing different visual state of the same element. This may be