[css-display] Typo: "it not" -> "it is not"

Author: Loirooriol

css-display/Overview.bs

@@ -979,7 +979,7 @@ Appendix A: Glossary</h2>
 		<dd>
 			A box is <a>out-of-flow</a> if it is floated (via 'float')
 			or absolutely-positioned (via ''absolute'' or ''position/fixed'' positioning).
-			A box is <a>in-flow</a> if it not <a>out-of-flow</a>.
+			A box is <a>in-flow</a> if it is not <a>out-of-flow</a>.
 
 			Note: some formatting contexts inhibit floating,
 			so that an element with ''float: left'' is not necessarily <a>out-of-flow</a>.