Replace 3_drude_model.md

Fixed a few errors in the excersises
- Different wording in excerise 4.3
- error in 4.4. E_x -> E_y

docs/3_drude_model.md

@@ -334,8 +334,8 @@ As before, we consider a 2D sample in the xy plane. We push a current through th
 
 1. Express the current density $\bf J$ as a function of the drift velocities $\mathbf{v_e}$ and $\mathbf{v_h}$ of the charge carriers. 
 2. Show that the drift velocities are given by $\mathbf{v_e} = -\mu_e(\mathbf{E} + \mathbf{v_e}\times\mathbf{B})$ and $\mathbf{v_h} = \mu_h(\mathbf{E} + \mathbf{v_h}\times\mathbf{B})$ 
-3. Assume that $E_x \gg |v_{e,y}| B_z$ and $E_x \gg |v_{h,y}| B_z$ to show that $J_x = eE_x(n_e\mu_e + n_h\mu_h)$. Discuss the two terms in this equation and why they should be added.
-4. Under the same assumption, show that $J_y = eE_x(n_e\mu_e + n_h\mu_h) + eB_zE_x(n_e\mu_e^2-n_h\mu_h^2)$. What should $J_y$ be equal to?
+3. Assume that $E_x \gg |v_{e,y}| B_z$ and $E_x \gg |v_{h,y}| B_z$ to show that $J_x = eE_x(n_e\mu_e + n_h\mu_h)$. Discuss the two terms of this last equation and why they should be added.
+4. Under the same assumption, show that $J_y = eE_y(n_e\mu_e + n_h\mu_h) + eB_zE_x(n_e\mu_e^2-n_h\mu_h^2)$. What should $J_y$ be equal to?
 5. Use the derived equations to show that the Hall electric field is
     $$ E_y = J_xB_z\frac{n_h\mu_h^2 - n_e\mu_e^2}{e(n_h\mu_h + n_e \mu_e)^2} $$
     Extract the Hall coefficient $R_H$ and discuss what determines its sign.