Update 13_semiconductors_solutions.md - typo

docs/13_semiconductors_solutions.md

@@ -200,7 +200,7 @@ Analoguous, we find for the hole density
 
 \begin{align}
 n_h &= \int_{0}^{\infty} f(E_h) g_h(E_h) dE_h\\
-&\approx \frac{\sqrt{\pi k_B T} e^{-\beta \mu}}{a \sqrt{t_{vb}}}. 
+&\approx \frac{\sqrt{k_B T} e^{-\beta \mu}}{\pi a \sqrt{t_{vb}}}. 
 \end{align}
 
 #### Question 5.