typo fix

docs/13_semiconductors_solutions.md

@@ -108,7 +108,7 @@ n_h \approx \int_{-E_v}^\infty\frac{m_h}{\pi\hbar^2}e^{-(E_h+E_F)/k_BT}dE_h = N_
 $$
 
 $$
-n_e \approx \int_{E_c}^\infty\frac{m_e}{\pi\hbar^2}e^{-(E-E_F)/k_BT}dE = N_C e^{-(E_c+E_F)/k_BT}
+n_e \approx \int_{E_c}^\infty\frac{m_e}{\pi\hbar^2}e^{-(E-E_F)/k_BT}dE = N_C e^{-(E_c-E_F)/k_BT}
 $$
 
 where