Update 3_drude_model.md - typo fix

src/3_drude_model.md

@@ -149,7 +149,7 @@ m⟨\mathbf{v}(t+dt)⟩ &= [m⟨\mathbf{v}(t)⟩ - e (\mathbf{E} + \mathbf{v} ×
                     &\quad\quad\quad\quad + e (\mathbf{E} + \mathbf{v} × \mathbf{B}) m⟨\mathbf{v}(t)⟩dt²/τ.
 \end{align}
 $$
-We now neglect the term proportional to $dt²$ (it vanishes when $dt → ∞$).
+We now neglect the term proportional to $dt²$ (it vanishes when $dt → 0$).
 Finally, we recognize that $(⟨\mathbf{v}(t+dt)⟩ - (⟨\mathbf{v}(t)⟩)/dt = d⟨\mathbf{v}(t)⟩)/dt$, and arrive to
 $$
 m\frac{d⟨\mathbf{v}⟩}{dt} = -m\frac{⟨\mathbf{v}⟩}{τ} -e\left(\mathbf{E}+⟨\mathbf{v}⟩×\mathbf{B}\right).