Fix Vector2Angle() not working as expected

[jdeokkim]

This pull request fixes Vector2Angle() not working as expected. (#2172, #2193)

Here's the animation of a simple example I made with the fixed version of Vector2Angle():

float angle = Vector2Angle((Vector2) { .x = 1 }, Vector2Subtract(GetMousePosition(), SCREEN_CENTER)) * RAD2DEG;

[raysan5]

@jdeokkim thank you very much for the review and the visual explanation. 👍🙂