Fix typo.

src/2_debye_model.md

@@ -150,7 +150,7 @@ There is therefore exactly one allowed ${\bf k}$ per volume $\left(\frac{2\pi}{L
 
 When we consider larger and larger box sizes $L→∞$, the volume per allowed mode becomes smaller and smaller, and eventually we obtain an integral:
 $$
-\sum_\mathbf{k}  \rightarrow \frac{L^3}{(2\pi)^3}\iiint\limits_{-∞}^{∞}dk_x dk_y dk_z )
+\sum_\mathbf{k}  \rightarrow \frac{L^3}{(2\pi)^3}\iiint\limits_{-∞}^{∞}dk_x dk_y dk_z
 $$