During the production process an error was introduced into equation (14 ). The absolute value symbol was moved to the wrong place. The correct version of equation (14 ) is listed below:
$\begin{eqnarray}\begin{array}{l}\left|\displaystyle \frac{{\partial }_{t}a(z,t)}{a(z,t)}\right|=\left|\displaystyle \frac{\tanh \left[\displaystyle \frac{1}{{z}_{0}}(z-z(0)-{V}_{g}t)\right]{V}_{g}}{{z}_{0}}-\displaystyle \frac{i}{2{L}_{D}}\right|\\ \,\leqslant \sqrt{\displaystyle \frac{{V}_{g}^{2}}{{z}_{0}^{2}}+\displaystyle \frac{1}{4{L}_{D}^{2}}},\end{array}\end{eqnarray}$