sign error fix

docs/14_doping_and_devices_solutions.md

@@ -107,7 +107,7 @@ This a "particle in a box" problem.
 
 \begin{align*}
 -\frac{\hbar^2}{2m_e^{\ast}} \nabla^2 \Psi_e &= (E_e-E_c)\Psi_e\\
--\frac{\hbar^2}{2m_h^{\ast}} \nabla^2 \Psi_h &= (-E_h-E_v)\Psi_h 
+-\frac{\hbar^2}{2m_h^{\ast}} \nabla^2 \Psi_h &= (E_h+E_v)\Psi_h 
 \end{align*}
 
 Here and below $E_h$ is the energy of a hole in the valence band.