Update AboutMutability.js

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.