[css-backgrounds-3] Use new #? multiplier in background

Author: cdoublev

css-backgrounds-3/Overview.bs

@@ -976,7 +976,7 @@ layer.
 
 <pre class="propdef">
 Name: background
-Value: [<<bg-layer>># ,]? <<final-bg-layer>>
+Value: <<bg-layer>>#? <<final-bg-layer>>
 Initial: see individual properties
 Applies to: all elements
 Inherited: no