[css-display][meta] fix id

Author: tabatkins

css-display-3/Overview.bs

@@ -56,7 +56,7 @@ Introduction</h2>
 	and renders it onto a <a href="https://www.w3.org/TR/CSS2/intro.html#canvas">canvas</a>
 	(such as your screen, a piece of paper, or an audio stream).
 	To do this, it generates an intermediary structure,
-	the <dfn id="box tree" oldids="css-box-tree" export for=CSS>box tree</dfn>,
+	the <dfn id="box-tree" oldids="css-box-tree" export for=CSS>box tree</dfn>,
 	which represents the formatting structure of the rendered document.
 	Each <dfn id="box" oldids="css-box" export for=CSS>box</dfn> in the <a>box tree</a>
 	represents its corresponding [=CSS/element=] (or <a>pseudo-element</a>)