fix power error

src/4_sommerfeld_model_solutions.md

@@ -198,6 +198,6 @@ $$
 4.
 
 $$
-C_v(T) = \frac{\partial E}{\partial T} = \frac{3Ak_B^2T^2}{\pi c^2}
+C_v(T) = \frac{\partial E}{\partial T} = \frac{3Ak_B^3T^2}{\pi c^2}
 $$