Update 2_debye_model_solutions.md - typo fix

docs/2_debye_model_solutions.md

@@ -95,7 +95,7 @@ ax.legend();
 1. The energy stored in the vibrational modes of a two-dimensional Debye solid is:
     
     \begin{align*}
-    E & = \int_0^{\infty}(n_B(\omega(\mathbf{k}))+\frac{1}{2})\hbar\omega(\mathbf{k}) d\mathbf{k} \\    
+    E & = 2_p \frac{L^2}{4\pi^2}\int_0^{\infty}(n_B(\omega(\mathbf{k}))+\frac{1}{2})\hbar\omega(\mathbf{k}) d\mathbf{k} \\    
     & = \frac{L^2}{\pi v^2\hbar^2\beta^3}\int_{0}^{\beta\hbar\omega_D}\frac{x^2}{e^{x} - 1}dx + E_0
     \end{align*}