Update 13_semiconductors.md - fix

src/13_semiconductors.md

 ```python
+
+A completely filled band carries no electric current:
+
+$$
+\begin{align}
 from matplotlib import pyplot
 
 import numpy as np
@@ -57,13 +62,8 @@ A completely filled band is very similar to a completely empty band.
 In a filled band $n(E)=1$ because $|E - E_F| \gg kT$. In an empty band $n(E)=0$.
 
 Heat capacity $C_v = \tfrac{d}{dT}\int_{-\infty}^\infty E\times g(E) \times dE\times n_F(E, T) = 0$.
-
-A completely filled band carries no electric current:
-
-$$
-\begin{align}
-j = 2e \frac{L}{2\pi} \int_{-\pi/a}^{\pi/a} v(k) dk = 2e \frac{L}{2\pi \hbar} \int_{-\pi/a}^{\pi/a} \frac{dE}{dk})\times dk = \\
-2e \frac{L}{2\pi \hbar} [E(-\pi/a) - E(\pi/a)] = 0
+j = 2e \frac{1}{2\pi} \int_{-\pi/a}^{\pi/a} v(k) dk = 2e \frac{1}{2\pi \hbar} \int_{-\pi/a}^{\pi/a} \frac{dE}{dk})\times dk = \\
+2e \frac{1}{2\pi \hbar} [E(-\pi/a) - E(\pi/a)] = 0
 \end{align}
 $$