fix typo in 5.5.2 article shuold be the, not a

Author: albertobottopoala

advection/advection-higherorder.tex

@@ -1087,7 +1087,7 @@ \subsection{Finite differences}
   - \frac{1}{\Delta x} \left \{ \Fb_{i+\myhalf} -
                                 \Fb_{i-\myhalf} \right \}.
 \end{equation}
-In the finite difference method a interpretation used is different. We are now
+In the finite difference method the interpretation used is different. We are now
 thinking of
 \begin{equation}
   \label{eq:fd-deriv}