[css-align] Fix for='' values.

Author: tabatkins

css-align/Overview.bs

@@ -261,7 +261,7 @@ Baseline Alignment: the ''baseline'' and ''last-baseline'' keywords</h3>
 
 	and are defined below:
 
-	<dl dfn-type=value dfn-for="justify-content, justify-self, justify-items, align-content, align-self, align-items">
+	<dl dfn-type=value dfn-for="justify-content, justify-self, justify-items, align-content, align-self, align-items, <baseline-position>">
 		<dt><dfn>baseline</dfn>
 		<dd>
 			Indicates <dfn dfn>first-baseline alignment</dfn>: