[css-flexbox] Restore definition of auto which was lost in previous commit. >_<;;

Author: fantasai

css-flexbox/Overview.bs

@@ -2004,6 +2004,12 @@ Cross-axis Alignment: the 'align-items' and 'align-self' properties</h3>
 	Values have the following meanings:
 
 	<dl dfn-type=value dfn-for="align-items, align-self">
+		<dt><dfn>auto</dfn>
+		<dd>
+			Defers <a>cross-axis</a> alignment control
+			to the value of 'align-items' on the parent box.
+			(This is the initial value of 'align-self'.)
+
 		<dt><dfn>flex-start</dfn>
 		<dd>
 			The <a>cross-start</a> margin edge of the <a>flex item</a>