Update index.mdx (#414)

Just changed the A21 element of the array.

public/content/lessons/2016/linear-transformations/index.mdx

@@ -243,7 +243,7 @@ If you're given a 2x2 matrix describing a linear transformation, and a specific
   image="./figures/matrices/MatrixNotation2x2InputOutput.svg" 
 />
 
-When you do this super generally, with a matrix that has entries $\left[\begin{array}{cc}a & b \\c & d\end{array}\right]$, and a vector $\left[\begin{array}{c} x \\ y \end{array}\right]$, what's the resulting vector? Remember here that the first column $\left[\begin{array}{c} a \\ b \end{array}\right]$ corresponds to the place where the first basis vector lands, and the second column $\left[\begin{array}{c} b \\ d \end{array}\right]$ is the place where the second basis vector lands.
+When you do this super generally, with a matrix that has entries $\left[\begin{array}{cc}a & b \\c & d\end{array}\right]$, and a vector $\left[\begin{array}{c} x \\ y \end{array}\right]$, what's the resulting vector? Remember here that the first column $\left[\begin{array}{c} a \\ c \end{array}\right]$ corresponds to the place where the first basis vector lands, and the second column $\left[\begin{array}{c} b \\ d \end{array}\right]$ is the place where the second basis vector lands.
 
 Well, it will be $x \left[\begin{array}{c} a \\ c \end{array}\right] + y \left[\begin{array}{c} b \\ d \end{array}\right]$ and, putting this together, you get a vector $\left[\begin{array}{c} ax + by \\ cx + dy \end{array}\right]$.