Update 12_band_structures_in_higher_dimensions_solutions.md - typo fix

src/12_band_structures_in_higher_dimensions_solutions.md

@@ -28,7 +28,7 @@ Empirical: Salt is transparent, Fermi level must be inside a large bandgap -> in
 
 ### Subquestion 1
 
-$$ E \phi_{n,m} = \varepsilon_0\phi_{n,m}-t_1 \left(\phi_{m,n-1}+\phi_{m,n+1}\right) -t_2 \left(\phi_{m-1,n}+\phi_{m+1,n}\right) $$
+$$ E \phi_{n,m} = \varepsilon_0\phi_{n,m}-t_1 \left(\phi_{n-1,m}+\phi_{n+1,m}\right) -t_2 \left(\phi_{n,m-1}+\phi_{n,m+1}\right) $$
 
 ### Subquestion 2