[css-tables-3]: ensure border-spacing is a non-negative length

This is true of all browsers today, but is not specified at a grammar
level.

This change ensures it is specified at a grammar level.

Author: keithamus

css-tables-3/Overview.bs

@@ -971,7 +971,7 @@ spec:css-display-4; type:property; text:display
 
 			<pre class='propdef'>
 				Name: border-spacing
-				Value: <<length>>{1,2}
+				Value: <<length [0,∞]>>{1,2}
 				Initial: 0px 0px
 				Inherited: yes
 				Applies To: <a>table grid boxes</a> when 'border-collapse' is ''border-collapse/separate''