Merge pull request patmorin#53 from caprice-j/binomial_typo

fixed a typo in a binomial identity

latex/intro.tex

@@ -735,7 +735,7 @@ \subsection{Randomization and Probability}
 \end{align*}
 This requires that we know enough to calculate that $\Pr\{X=i\}
 = \binom{k}{i}/2^k$, and that we know the binomial identities
-$i\binom{k}{i}=k\binom{k-1}{i}$ and $\sum_{i=0}^{k} \binom{k}{i} = 2^{k}$.
+$i\binom{k}{i}=k\binom{k-1}{i-1}$ and $\sum_{i=0}^{k} \binom{k}{i} = 2^{k}$.
 
 Using indicator variables and linearity of expectation makes things
 much easier.  For each $i\in\{1,\ldots,k\}$, define the indicator