[css-backgrounds-4] Made 'none' in 'box-shadow-offset' repeatable (w3c#8580)

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -766,7 +766,7 @@ layer.
 
 <pre class="propdef">
 Name: box-shadow-offset
-Value: none | <<length>>{2}#
+Value: [ none | <<length>>{2} ]#
 Initial: none
 Applies to: all elements
 Inherited: no