Update 2015-10-20-math.md (alshedivat#514)

Typo

_posts/2015-10-20-math.md

@@ -18,7 +18,7 @@ You can also use `\begin{equation}...\end{equation}` instead of `$$` for display
 MathJax will automatically number equations:
 
 \begin{equation}
-\label{eq:caushy-shwarz}
+\label{eq:cauchy-schwarz}
 \left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right)
 \end{equation}