Fix Switch statement in swift.md

[TRDSCSH]

补全 Switch 分支。

参考代码：https://www.codecademy.com/resources/docs/swift/switch

var secondaryColor = "green"

switch secondaryColor {
  case "orange":
    print("Mix of red and yellow.")
  case "green":
    print("Mix of blue and yellow.")
  case "purple":
    print("Mix of red and blue.")
  default:
    print("This might not be a secondary color.")
}

// Output: Mix of blue and yellow.