fix a typo in superscript of rhs expansion for implicit methods

closes issue #2

Author: zingale

intro/intro.tex

@@ -478,7 +478,7 @@ \subsubsection{Implicit methods}
 \end{equation}
 Using this approximation, we can expand the righthand side vector,
 \begin{equation}
-{\bf f}(t^{n+1},{\bf y}^{n+1}) = {\bf f}(t^n, {\bf y}^{n+1}_0) +
+{\bf f}(t^{n+1},{\bf y}^{n+1}) = {\bf f}(t^{n+1}, {\bf y}^{n+1}_0) +
      \left . \frac{\partial {\bf f}}{\partial {\bf y}} \right |_0 \Delta {\bf y}_0 + \ldots 
 \end{equation} 
 Here we recognize the Jacobin matrix, ${\bf J} \equiv \partial {\bf