fix +1 in the fermi function - exercise 3.1

src/4_sommerfeld_model_solutions.md

@@ -86,7 +86,7 @@ Total energy: $E = \int_{0}^{\infty}g(E)f(E)EdE $
 1.
 
 $$
-E = \int_{0}^{\infty}\epsilon g(\epsilon)f(\epsilon)d\epsilon = 2.10^{10}\int_{0}^{\infty}\frac{\epsilon^{\frac{3}{2}}}{e^\frac{\epsilon-5.2}{k_BT}}d\epsilon
+E = \int_{0}^{\infty}\epsilon g(\epsilon)f(\epsilon)d\epsilon = 2.10^{10}\int_{0}^{\infty}\frac{\epsilon^{\frac{3}{2}}}{e^\frac{\epsilon-5.2}{k_BT}+1}d\epsilon
 $$
 
 2.