fix math

src/8_differential_equations_2.md

@@ -60,7 +60,7 @@ y_1 ' = & y_2 \\
 y_2 ' = & y_3 \\
 \vdots & \\
 y_{n-1} ' = & y_{n} \\
-$$y_{n} ' = & - a_{0} y_{1} - a_{1} y_{2} - \cdots - a_{n-1} y_{n}.
+y_{n} ' = & - a_{0} y_{1} - a_{1} y_{2} - \cdots - a_{n-1} y_{n}.
 \end{split}$$
 
 Notice that together these $n$ equations form a linear first order system, the