Update index.mdx (#412)

Changed a + to a - in the 3x3 determinant section.

public/content/lessons/2016/determinant/index.mdx

@@ -311,7 +311,7 @@ $$
 \color{red}h&\color{blue}i
 \end{bmatrix}
 \right)
-\\ \color{black}&-
+\\ \color{black}&+
 \color{red}b
 \color{black}\det\left(
 \begin{bmatrix}
@@ -358,4 +358,4 @@ Since applying two matrices sequentially leads to a new linear transformation, i
 
 </FreeResponse>
 
-[^1]: Whenever I read the word "parallelepiped" I hear the voice of one of my favorite professors, Yakov Eliashberg.
+[^1]: Whenever I read the word "parallelepiped" I hear the voice of one of my favorite professors, Yakov Eliashberg.