Update Testingclasses.rst

The sentence read:  "A method like move does its work by changing the contents of a mutable object (the point instance has its instance variable changes) so it needs to be tested with a side effect test."

I changed the typo of "(the point instance has its instance variable changes)" to "(the point instance has its instance variable changed)"

The sentence now reads:  "A method like move does its work by changing the contents of a mutable object (the point instance has its instance variable changed) so it needs to be tested with a side effect test."