[css-display] Fix swapped terminology error

Author: fantasai

css-display/Overview.bs

@@ -799,7 +799,7 @@ Appendix A: Glossary</h2>
 		<dt><dfn>out-of-flow</dfn>
 		<dt><dfn>in-flow</dfn>
 		<dd>
-			A box is <a>in-flow</a> if it is floated (via 'float')
+			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>.