indentation fix

src/2_debye_model.md

@@ -303,9 +303,9 @@ cbar.set_label(r'$|\psi^2|$')
 
 1. Describe how many phonons in which $k$-state this solid has. Explain your answer.
 
-   ??? hint
+    ??? hint
 
-       There are $n=2$ phonons in the state with $k=4\pi/L$ and $n=2$ phonons in a state with $k=-4\pi/L$.
+        There are $n=2$ phonons in the state with $k=4\pi/L$ and $n=2$ phonons in a state with $k=-4\pi/L$.
 
 2. Describe the concept of k-space. What momenta are allowed in a 2D system with dimensions $L\times L$?
 3. Explain the concept of density of states.