fix typo (- instead of +). Closes issue #2

TODO

@@ -154,3 +154,9 @@ http://tex.stackexchange.com/questions/172475/how-can-i-define-a-custom-tcolorbo
 
 
 http://www.scriptscoop2.com/t/e6de0140d668/counters-for-chapter-and-section-environments-when-using-tcolorbox-for.html
+
+
+http://tex.stackexchange.com/questions/254401/use-the-exercise-environment-from-legrand-book-template
+
+http://www.latextemplates.com/template/the-legrand-orange-book
+

advection/advection.tex

@@ -941,7 +941,7 @@ \section{Multi-dimensional advection}
 the average over the face of the zone with the flux at the center of the face.  Then we have:
 \begin{equation}
 a_{i,j}^{n+1} = a_{i,j}^n - \Delta t \left [
-   \frac{(ua)_{i+\myhalf,j}^{n+\myhalf} - (ua)_{i-\myhalf,j}^{n+\myhalf}}{\Delta x} -
+   \frac{(ua)_{i+\myhalf,j}^{n+\myhalf} - (ua)_{i-\myhalf,j}^{n+\myhalf}}{\Delta x} +
    \frac{(va)_{i,j+\myhalf}^{n+\myhalf} - (va)_{i,j-\myhalf}^{n+\myhalf}}{\Delta y} \right ]
 \end{equation}