[css-display-3] Wording tweak

Author: fantasai

css-display-3/Overview.bs

@@ -111,7 +111,7 @@ Introduction</h2>
 	In constructing the box tree,
 	boxes generated by an element are descendants of
 	the principal box of any ancestor elements.
-	In general,
+	In the general case,
 	the direct <dfn export for=CSS>parent box</dfn>
 	of an element’s <a>principal box</a>
 	is the <a>principal box</a>