Update Overview.bs

Author: zhangenming

css-align-3/Overview.bs

@@ -1961,7 +1961,7 @@ Block-Axis (or Cross-Axis) Default Alignment: the 'align-items' property</h3>
 
 	<pre class="propdef">
 	Name: align-items
-	Value: normal | stretch | <<baseline-position>> | [ <<overflow-position>>? <<self-position>> ]
+	Value: normal | stretch | <<baseline-position>> | <<overflow-position>>? <<self-position>>
 	Initial: normal
 	Applies to: all elements
 	Inherited: no