[css-values-4] Forgot to change the production rule for round(). w3c#9668

Author: tabatkins

css-values-4/Overview.bs

@@ -4225,7 +4225,7 @@ Syntax</h3>
 	<<min()>>   = min( <<calc-sum>># )
 	<<max()>>   = max( <<calc-sum>># )
 	<<clamp()>> = clamp( [ <<calc-sum>> | none ], <<calc-sum>>, [ <<calc-sum>> | none ] )
-	<<round()>> = round( <<rounding-strategy>>?, <<calc-sum>>, <<calc-sum>> )
+	<<round()>> = round( <<rounding-strategy>>?, <<calc-sum>>, <<calc-sum>>? )
 	<<mod()>>   = mod( <<calc-sum>>, <<calc-sum>> )
 	<<rem()>>   = rem( <<calc-sum>>, <<calc-sum>> )
 	<<sin()>>   = sin( <<calc-sum>> )