[css-grid] fix markup

Author: fantasai

css-grid/Overview.bs

@@ -1246,13 +1246,13 @@ Explicit Track Sizing: the 'grid-template-rows' and 'grid-template-columns' prop
 			Represents the largest <a>min-content contribution</a>
 			of the  <a>grid items</a> occupying the <a>grid track</a>.
 
-		<dt><dfn lt="minmax()">minmax(min, max)</dfn>
+		<dt><dfn lt="minmax()">minmax(<var>min</var>, <var>max</var>)</dfn>
 		<dd>
 			Defines a size range
 			greater than or equal to <var>min</var>
 			and less than or equal to <var>max</var>.
 			If <var>max</var> &lt; <var>min</var>,
-			then <var>max</var> is ignored and ''minmax(min,max)'' is treated as <var>min</var>.
+			then <var>max</var> is ignored and ''minmax(<var>min</var>,<var>max</var>)'' is treated as <var>min</var>.
 			As a maximum, a <<flex>> value sets the track's <a>flex factor</a>;
 			it is invalid as a minimum.