fix integration limit

src/4_sommerfeld_model.md

@@ -233,7 +233,7 @@ Because the dispersion of the free electron is isotropic[^2], we utilize spheric
 The total number of states is then
 $$
 \begin{align}
-N &\overset{\mathrm{3D}}{=}2_s \left(\frac{L}{2\pi}\right)^3\int_{\varepsilon(\mathbf{k} < \varepsilon)}\mathrm{d}{\mathbf{k}}\\
+N &\overset{\mathrm{3D}}{=}2_s \left(\frac{L}{2\pi}\right)^3\int_{\varepsilon(\mathbf{k}) < \varepsilon}\mathrm{d}{\mathbf{k}}\\
 &=2_s \left(\frac{L}{2\pi}\right)^3 4\pi\int k^2\mathrm{d}k\\
 &=\frac{V}{\pi^2}\int k^2\mathrm{d}k,
 \end{align}