Update 2_debye_model.md - typo fix

src/2_debye_model.md

@@ -127,7 +127,7 @@ The easiest option people have invented so far is a box $L×L×L$ with **periodi
 Periodic boundary conditions imply that the atomic displacement $\mathbf{\delta r}$ is periodic. Let's consider a displacement by $L$ in the $x$-direction
 
 $$
-\mathbf{\delta r}(\mathbf{r} + L\mathbf{\hat{x}}) &= \mathbf{\delta r}(\mathbf{r})
+\mathbf{\delta r}(\mathbf{r} + L\mathbf{\hat{x}}) = \mathbf{\delta r}(\mathbf{r})
 $$
 To satisfy this equation, we arrive at the condition $k_x=p 2 \pi/L$]$, with $p= ..., -2, -1, 0, 1, 2$ in $\mathbb{Z}$.