Update abstract class example in class-modifiers.md

[av153k]

The example for abstract classes has an issue. I think it's a typo. The extending class Car does not override the abstract method moveForward from abstract class Vehicle. So I just made the method moveForward a normal method rather than abstract method that it was. If suggested, I can also change the Car class to implement the original abstract moveForward method. But I think making it normal, will clarify the difference between extends and implements.

Fixes

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