Fix Prototype-Pattern SampleCode

I want to modify the prototype pattern sample code.
By default, the structure thinks that it does not need to use a prototype pattern because the values are copied.
The sample code is structured, so I think it is right to change it to a class.

Author: sookim-1

README.md

@@ -1058,7 +1058,7 @@ This practise is particularly useful when the construction of a new object is in
 ### Example
 
 ```swift
-struct MoonWorker {
+class MoonWorker {
 
     let name: String
     var health: Int = 100