Fix typo in privacy/security section.

Author: khushalsagar

css-view-transitions-1/Overview.bs

@@ -1137,7 +1137,7 @@ The {{ViewTransition/domUpdated}} [=getter steps=] are to return [=this's=] [=Vi
 Privacy and Security Considerations</h2>
 
 The images generated using [=capture the image=] algorithm could contain cross-origin data if the Document is embedding cross-origin resources. The
-implementations must sure this data can not be accessed by the Document. This should be feasible since access to this data should already be prevented
+implementations must ensure this data can not be accessed by the Document. This should be feasible since access to this data should already be prevented
 in the default rendering of the Document.
 
 This feature introduces no privacy considerations.