adds dollar signs to create equation

docs/12_band_structures_in_higher_dimensions_solutions.md

@@ -86,12 +86,14 @@ Four in total: $(\pm\pi/a,\pm\pi/a)$.
 ### Question 3.
 
 Define a basis, e.g.
+$$
 \begin{align*}
     \left|0\right\rangle &= (\pi/a,\pi/a) \\
     \left|1\right\rangle &= (\pi/a,-\pi/a) \\
     \left|2\right\rangle &= (-\pi/a,-\pi/a) \\
     \left|3\right\rangle &= (-\pi/a,\pi/a)
 \end{align*}
+$$
 The Hamiltonian becomes
 
 $$