Added link to calc-background-position-003.html into css-values-4/Overview.bs

Author: TalbotG

css-values-4/Overview.bs

@@ -3320,6 +3320,7 @@ Serialization</h3>
 	css/css-values/calc-serialization-002.html
 	css/css-values/getComputedStyle-border-radius-001.html
 	css/css-values/getComputedStyle-border-radius-003.html
+	css/css-values/calc-background-position-003.html
 	</wpt>
 
 	<div algorithm="serialize a min()/max() value">