Attempt to fix equation

docs/12_band_structures_in_higher_dimensions_solutions.md

@@ -86,14 +86,15 @@ Four in total: $(\pm\pi/a,\pm\pi/a)$.
 ### Question 3.
 
 Define a basis, e.g.
-$$
-\begin{align*}
-    \left|0\right\rangle &= (\pi/a,\pi/a) \\
-    \left|1\right\rangle &= (\pi/a,-\pi/a) \\
-    \left|2\right\rangle &= (-\pi/a,-\pi/a) \\
-    \left|3\right\rangle &= (-\pi/a,\pi/a)
-\end{align*}
-$$
+\begin{equation*}
+    \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*}
+\end{equation*}
+
 The Hamiltonian becomes
 
 $$