typo

src/homework_solutions/lecture_1_solutions.md

@@ -42,7 +42,7 @@ For the "lock-in" measurement, the situation is different.  The filter of the lo
 
 $$
 \begin{align}
-\sigma_v^2 = \int_{f_1}^{f_2} S_vv (f) df \approx S_{vv} (f_0) \Delta f
+\sigma_v^2 = \int_{f_1}^{f_2} S_{vv} (f) df \approx S_{vv} (f_0) \Delta f
 \end{align}
 $$