make notation consistent (-t instead of +t)

src/12_band_structures_in_higher_dimensions.md

@@ -135,7 +135,7 @@ Observe that the top of the first band is above the bottom of the lowest band. T
 A larger $V$ makes the Fermi surface more distorted and eventually makes the material insulating.
 Let's compare the almost parabolic dispersion of the nearly free electron model with a tight-binding model in 2D.
 
-We now have a dispersion relation $E = E_0 + 2t(\cos k_x a + \cos k_y a)$, which looks like this:
+We now have a dispersion relation $E = E_0 - 2t(\cos k_x a + \cos k_y a)$, which looks like this:
 
 ```python
 momenta = np.linspace(-pi, pi, 100)