[css-viewport-1] Hardcode numeric range in syntax

Author: cdoublev

css-viewport/Overview.bs

@@ -398,7 +398,7 @@ Note: 'zoom' does not affect or prevent 'transform' scaling.
 
 <pre class="propdef">
 	Name: zoom
-	Value: <<number>> || <<percentage>>
+	Value: <<number [0,∞]>> || <<percentage [0,∞]>>
 	Initial: 1
 	Applies to: all <<length>> property values of all elements
 	Inherited: no