docs: fix invalid return value in "Returning errors" example. (#199)

Author: ffenix113

docs/Custom-Functions.md

@@ -154,7 +154,7 @@ func main() {
 			if i < 0 {
 				return 0, errors.New("value cannot be less than zero")
 			}
-			return i * 2
+			return i * 2, nil
 		},
 	}