fixes md list issue

src/2_debye_model_solutions.md

@@ -36,7 +36,7 @@ $$
 2. See plot below (shown for $T_{D,1} < T_{D,2}$)
 3. The polarization is related to the direction of the amplitudes of the waves with respect to the direction of the wave.
 In 3D, there are only 3 different amplitude directions possible.
-4.
+4. 
 $$
 \int k_x k_y \rightarrow \int_{0}^{2\pi} \mathrm{d} \theta \int_{0}^{\infty} k \mathrm{d} k = 2\pi \int_{0}^{\infty} k \mathrm{d} k 
 $$