Update 3_drude_model.md - typo fix

docs/3_drude_model.md

@@ -160,7 +160,7 @@ To find the average velocity, we take a weighted average of these two groups of
 \begin{align*}
 m\mathbf{v}(t+dt)  &= [m\mathbf{v}(t) + F dt]\left(1 - \frac{dt}{\tau}\right) + 0⋅\frac{dt}{\tau} \\
   &= [m\mathbf{v}(t) + \mathbf{F} dt] \left(1 - \frac{dt}{\tau}\right) \\
-                  &= m\mathbf{v}(t) + dt [\mathbf{F} - m\frac{\mathbf{v(t)}}{\tau}] - \frac{\mathbf{F}}{\tau} dt^2
+                  &= m\mathbf{v}(t) + dt [\mathbf{F} - m\frac{\mathbf{v}(t)}{\tau}] - \frac{\mathbf{F}}{\tau} dt^2
 \end{align*}
 
 We now neglect the term proportional to $dt^2$ (it vanishes when $dt → 0$).