typo fix

code/band_structures.py

@@ -82,6 +82,7 @@ def tight_binding_1d():
 
 def meff_1d_tb():
     pyplot.figure(figsize=(8, 5))
+    k = np.linspace(-pi, pi, 300)
     meff = 1/np.cos(k)
     color = list(matplotlib.rcParams['axes.prop_cycle'])[0]['color']
     pyplot.plot(k[meff > 0], meff[meff > 0], c=color)