[css-sizing] Fix link.

Author: fantasai

css-sizing-3/Overview.bs

@@ -285,7 +285,7 @@ New Keywords for 'width' and 'height'</h3>
 		<dt><dfn>fit-content(<<length-percentage>>)</dfn>
 		<dd>
 			Use the fit-content formula
-			with the <a>available size</a> replaced by the specified argument,
+			with the <a>available space</a> replaced by the specified argument,
 			i.e.
 			<code>min(<a>max-content size</a>, max(<a>min-content size</a>, <<length-percentage>>))</code>
 	</dl>