[css-multicol-2] Improve columns syntax

Author: rachelandrew

css-multicol-2/Overview.bs

@@ -947,7 +947,7 @@ The number and size of columns</h2>
 
 	<pre class="propdef shorthand">
 	Name: columns
-	Value: <<'column-width'>> || <<'column-count'>> / <<'column-height'>>?
+	Value: <<'column-width'>> || <<'column-count'>> [ / <<'column-height'>> ]?
 	</pre>
 
 	This is a shorthand property for setting 'column-width', 'column-count',