[css-rhythm] Fix computed value to handle 'none' correcty

Follow up fix to w3c#938.

Author: kojiishi

css-rhythm/Overview.bs

@@ -64,7 +64,7 @@ Adjusting Line Box Heights: the 'line-height-step' property {#line-height-step}
 	Animatable: no
 	Percentages: N/A
 	Media: visual
-	Computed Value: the absolute length for length
+	Computed Value: the absolute length for positive length, none otherwise
 	</pre>
 
 	This property defines the <dfn>step unit</dfn> for line box heights.