Fix mean_second_derivative_central docstring formula (was double the computed value)

[AlejandroCoronadoN]

The mean_second_derivative_central docstring's LaTeX formula does not match what the function computes. The formula carries an extra inner factor of 1/2, so the documented expression evaluates to exactly half of the returned value.

For x = [1, 3, 7, -3] the function returns -3.0 (asserted in test_feature_calculations.py), while the docstring's formula evaluates to -1.5.

The code is correct and covered by that test; only the docstring is stale. The extra 1/2 is a leftover from the 2019 refactor (#587) that rewrote the outer denominator to 1/(2(n-2)) but left the old inner 1/2 in place. This removes the redundant 1/2 and tightens the sum's upper index from n-1 to n-2, the number of valid central-difference terms.