Update 2_debye_model.md - another typo

src/2_debye_model.md

@@ -129,7 +129,7 @@ Periodic boundary conditions imply that the atomic displacement $\mathbf{\delta
 $$
 \mathbf{\delta r}(\mathbf{r} + L\mathbf{\hat{x}}) = \mathbf{\delta r}(\mathbf{r})
 $$
-To satisfy this equation, we arrive at the condition $k_x=p 2 \pi/L]$, with $p= ..., -2, -1, 0, 1, 2$ in $\mathbb{Z}$.
+To satisfy this equation, we arrive at the condition $k_x=p 2 \pi/L$, with $p= ..., -2, -1, 0, 1, 2$ in $\mathbb{Z}$.
 
 We see that periodicity implies that not all the points in $k$-space are allowed.
 Instead only waves for which each component $k_x, k_y, k_z$ of the $\mathbf{k}$-vector belongs to the set