Update 3_drude_model.md

docs/3_drude_model.md

@@ -298,7 +298,7 @@ Express the longitudinal resistance $R=V/I$, where $V$ is the voltage difference
 
 ### Exercise 2: Motion of an electron in a magnetic and an electric field.
 Consider an electron in free space experiencing a magnetic field $\mathbf{B}$ along the $z$-direction.
-Assume that the electron starts at the origin with a velocity $v_0$ along the $x$-direction.
+%Assume that the electron starts at the origin with a velocity $v_0$ along the $x$-direction.
 
 #### Question 1.
 Write down the Newton's equation of motion for the electron, compute $\frac{d\mathbf{v}}{{dt}}$.