Update 3_drude_model.md - clarification in exc 4

src/3_drude_model.md

@@ -311,7 +311,7 @@ We apply a magnetic field $\bf B$ along the $z$-direction to a current carrying
 
 $$\mathbf{E} = \begin{pmatrix} ρ_{xx} & ρ_{xy} \\ ρ_{yx} & ρ_{yy} \end{pmatrix} \mathbf{j}$$
 
-  1. Sketch $ρ_{xx}$ and $ρ_{xy}$ as a function of the magnetic field $\bf{B}$.
+  1. Sketch the expressions for $ρ_{xx}$ and $ρ_{xy}$ derived in the lecture notes as a function of the magnetic field $\bf{B}$.
   2. Invert the resistivity matrix to obtain the conductivity matrix,
 
      $$ \begin{pmatrix} \sigma_{xx} & \sigma_{xy} \\ \sigma_{yx} & \sigma_{yy} \end{pmatrix}$$