remove redundant px dimension from line-height-step initial value

Author: keithamus

css-rhythm-1/Overview.bs

@@ -356,7 +356,7 @@ Adjusting Line Box Heights: the 'line-height-step' property {#line-height-step}
 	<pre class='propdef'>
 	Name: line-height-step
 	Value: <<length [0,∞]>>
-	Initial: 0px
+	Initial: 0
 	Applies to: block containers
 	Inherited: yes
 	Percentages: N/A