fix: Fixed calculateCost() method in Food entity

- applied multiply() method instead type manual castings

src/main/java/com/f_lab/la_planete/domain/Food.java

@@ -47,7 +47,7 @@ public static Food of(BigDecimal price, int totalQuantity) {
   }
 
   public BigDecimal calculateCost(int quantity) {
-    return BigDecimal.valueOf((long) price.intValue() * quantity);
+    return price.multiply(BigDecimal.valueOf(quantity));
   }
 
   public void minusQuantity(int deductions) {