Update index.md

fixed typo on slide 10

06_StatisticalInference/05_Variance/index.md

@@ -98,7 +98,7 @@ the population variance
 ## To summarize
 - The sample variance, $S^2$, estimates the population variance, $\sigma^2$
 - The distribution of the sample variance is centered around $\sigma^2$
-- The the variance of sample mean is $\sigma^2 / n$
+- The variance of the sample mean is $\sigma^2 / n$
   - Its logical estimate is $s^2 / n$
   - The logical estimate of the standard error is $S / \sqrt{n}$
 - $S$, the standard deviation, talks about how variable the population is