[css-display] add missing word

Author: dauwhe

css-display/Overview.bs

@@ -808,7 +808,7 @@ Appendix A: Glossary</h2>
 
 			<p class="note">
 				Exclusions are able to affect content across formatting context boundaries.
-				(At time of writing, they are the layout feature that can.)
+				(At time of writing, they are the only layout feature that can.)
 				[[CSS3-EXCLUSIONS]]
 
 		<dt><dfn>block formatting context</dfn>