Remove redundant "dx += accel"

[Zelpuz]

In Chapter 1, under "Exercise: The Effect of Acceleration", the solution uses the closed-form solution to the differential equations for acceleration, velocity, and displacement.

def gen_data(x0, dx, count, noise_factor, accel=0.):
    zs = []
    for i in range(count):
        zs.append(x0 + accel * (i**2) / 2 + dx*i + randn()*noise_factor)
        dx += accel
    return zs

However, because this solution was used, there is no need to increment dx; in this solution, dx should just remain as its initial value (which we could call dx0, but I did not change it here).

See #381, #369, and #301. This problem appears to have arisen from the commit associated with #301, where the equation was changed from an iterative method to the closed-form solution, but the dx += accel was not removed.

This does not at all impact the lesson being taught in this section, but it might be confusing for readers if they cannot replicate the solution, especially since the solution uses zero noise.

[smcardle14]

Note that the text in this section should be reworded to be consistent with this change (especially the paragraph before and after the modified cell).