Fix typo in Madgwick filter documentation

[nbelakovski]

$J_{g,b}$ should be a 6x4 matrix, so that its transpose is 4x6, which can then be multiplied by the 6 element $f_{g,b}$ vector.

More importantly than just the dimensions, $J_{g,b}$ should look like this:

$$\begin{bmatrix} \frac{\partial f_{g_1}}{\partial q_w} & \frac{\partial f_{g_1}}{\partial q_x} & ... & \frac{\partial f_{g_1}}{\partial q_z} \\ ... \\ \frac{\partial f_{b_3}}{\partial q_w} & \frac{\partial f_{b_3}}{\partial q_x} & ... & \frac{\partial f_{b_3}}{\partial q_z} \\ \end{bmatrix}$$