fix the units in Drude solution 4.3

src/3_drude_model_solutions.md

@@ -126,7 +126,7 @@ Refer to the lecture notes
 3.
 
 $$
-\sigma_{xx} \approx \frac{1}{1 + \rho_{xy}^2}
+\sigma_{xx} \approx \frac{\rho_{yy}}{1 + \rho_{xy}^2}
 $$
 
 $$