more sign fixes

docs/14_doping_and_devices_solutions.md

@@ -116,7 +116,7 @@ Here and below $E_h$ is the energy of a hole in the valence band.
 
 \begin{align*}
 E_e = E_c + \frac{\hbar^2}{2m_e^{\ast}} ((\frac{\pi n}{L})^2+k_x^2+k_y^2),\\
--E_h = E_v - \frac{\hbar^2}{2m_h^{\ast}} ((\frac{\pi n}{L})^2+k_x^2+k_y^2)
+E_h = -E_v + \frac{\hbar^2}{2m_h^{\ast}} ((\frac{\pi n}{L})^2+k_x^2+k_y^2)
 \end{align*}
 
 ### Question 4.