[css-grid] Fix w3c#369.

Author: fantasai

css-grid/Overview.bs

@@ -3684,7 +3684,9 @@ Track Sizing Terminology</h3>
 
 		<dt><dfn>free space</dfn>
 		<dd>
-			Equal to the <a>available grid space</a> minus the sum of the <a>base sizes</a> of all the grid tracks, floored at zero.
+			Equal to the <a>available grid space</a> minus
+			the sum of the <a>base sizes</a> of all the grid tracks (including gutters),
+			floored at zero.
 			If <a>available grid space</a> is <a>indefinite</a>,
 			the <a>free space</a> is <a>indefinite</a> as well.