Update AboutMutability.js

[shaohua]

In this particular test case, the function on Person.prototype is not mutated. Rather, it is because the object aPerson itself has the method with the same name getFullName, it doesn't need to go to its prototype to find the method anymore.

In summary, both aPerson.getFullName and aPerson.prototype.getFullName exist. The getFullName on the prototype is masked, not mutated.