Update 13_semiconductors.md - moved other part of eq for j to 2nd line

src/13_semiconductors.md

@@ -62,8 +62,8 @@ A completely filled band carries no electric current:
 
 $$
 \begin{align}
-j = 2e \frac{L}{2\pi} \int_{-\pi/a}^{\pi/a} v(k) dk = \\
-2e \frac{L}{2\pi \hbar} \int_{-\pi/a}^{\pi/a} (dE/dk)\times dk = 2e \frac{L}{2\pi \hbar} [E(-\pi/a) - E(\pi/a)] = 0
+j = 2e \frac{L}{2\pi} \int_{-\pi/a}^{\pi/a} v(k) dk = 2e \frac{L}{2\pi \hbar} \int_{-\pi/a}^{\pi/a} (dE/dk)\times dk = \\
+2e \frac{L}{2\pi \hbar} [E(-\pi/a) - E(\pi/a)] = 0
 \end{align}
 $$