Update 12_band_structures_in_higher_dimensions.md - added minus sign in hopping

src/12_band_structures_in_higher_dimensions.md

@@ -215,7 +215,7 @@ Using the [periodic table](../fermi_surfaces) of the Fermi surfaces (or the stat
 ### Exercise 2: Tight-binding in 2D
 
 Consider a rectangular lattice with lattice constants $a_x$ and $a_y$.
-Suppose the hopping parameters in the two directions to be $t_1$ and $t_2$.
+Suppose the hopping parameters in the two corresponding directions to be $-t_1$ and $-t_2$.
 Consider a single orbital per atom and only nearest-neighbour interactions.
 
 1. Write down a 2D tight-binding Schrödinger equation (expand to 2D the results of 1D).