Fix error in exercise

Signed-off-by: Marcello Seri <marcello.seri@gmail.com>

7-integration.tex

@@ -185,7 +185,8 @@ \section{Orientation on manifolds}
 \end{definition}
 
 \begin{exercise}
-  Show that a diffeomorphism $F: M \to N$ is \emph{orientation preserving} if and only if for all $p\in M$, $dF_p : T_p M \to T_{F(p)}$ is an orientation preserving as a linear map, that is, if $\det(dF_p)>0$.
+  Let $M$ be a positively oriented smooth manifold.
+  Show that a diffeomorphism $F: M \to N$ is \emph{orientation preserving} if and only if for all $p\in M$, $dF_p : T_p M \to T_{F(p)} N$ is orientation preserving as a linear map, that is, if the Jacobian determinant of $F(p)$ with respect to some (and hence any) charts in the atlas is positive.
 \end{exercise}
 
 % \begin{theorem}