Use "-=" gives wrong result and does not pass the test

Sorry, I just remembered that they fixed the test cases for update_parameters in Week3 Planar Data to fix this problem: they no longer fail if your code modifies the global data.

@donnie1123: That must mean you do not have the latest version of the tests for this assignment. You should follow the “get latest version” procedure that is documented on the FAQ Thread. Please read the instructions all the way down to the bottom where it explains how to delete the public_tests.py and other python support files, so that you make sure to get the latest.