Added forgotten indices in 2.2

src/12_band_structures_in_higher_dimensions_solutions.md

@@ -32,7 +32,7 @@ $$ E \phi_{n,m} = \varepsilon_0-t_1 \left(\phi_{m,n-1}+\phi_{m,n+1}\right) -t_2
 
 ### Subquestion 2
 
-$$ \psi_n(\mathbf{r}) = u_n(\mathbf{r})e^{i\mathbf{k}\cdot\mathbf{r}} \quad \leftrightarrow \quad \phi_{n,m} = \phi_0 e^{i(k_x a_x + k_y a_y)}$$ 
+$$ \psi_n(\mathbf{r}) = u_n(\mathbf{r})e^{i\mathbf{k}\cdot\mathbf{r}} \quad \leftrightarrow \quad \phi_{n,m} = \phi_0 e^{i(k_x n a_x + k_y m a_y)}$$ 
 
 ### Subquestion 3