fix dimensions in counting electrons

src/12_band_structures_in_higher_dimensions.md

@@ -38,7 +38,7 @@ How many electrons may an insulator have per unit cell? To answer this we need t
 
 For a single band
 
-$$ N = 2 \int_{BZ}dk_x dk_y dk_z [L\times W\times H] (2\pi)^{-3} = 2 LWH $$
+$$ N = 2 \int_{BZ}dk_x dk_y dk_z [L\times W\times H] (2\pi)^{-3} = 2 LWH / a^3 $$
 
 So a single band has 2 electrons per unit cell (because of spin).