fixed typo

book/time_series/acf.md

@@ -249,7 +249,7 @@ And
 
 $$
 \hat{C}_1
-= \frac{\sum_{i=1}^{m} S_i S_{i+1}}{m-1}
+= \frac{\sum_{i=1}^{m-1} S_i S_{i+1}}{m-1}
 = \frac{2(1) + 1(0) + 0(-1) + (-1)(-2)}{5 - 1}
 = \frac{2 + 0 + 0 + 2}{4}
 = 1