Provide some more documentation.

diff --cc include/deal.II/grid/manifold.h
index ad79cd4f2e2fc381e1b73f48332700e23e038adb,7e7d2b143bff8c9a23b18852ecd51937206e82c4..f25be5896e5e3e24d0796ab815ebefac29787f1e
--- 1/include/deal.II/grid/manifold.h
--- 2/include/deal.II/grid/manifold.h
+++ b/include/deal.II/grid/manifold.h
@@@ -350,10 -353,10 +353,10 @@@ private
   *   for which you have explicit pull_back and push_forward
   *   transformations.
   *
-  *   This is an helper class which is useful when you have an explicit
+  *   This is a helper class which is useful when you have an explicit
   *   map from an Euclidean space of dimension chartdim to an Euclidean
   *   space of dimension spacedim which represents your manifold, i.e.,
 - *   when your manifold \f$\mathcal{M}\f$ can be represented by a map
 + *   when your manifold $\mathcal{M}$ can be represented by a map
   *   \f[
   *   F: \mathcal{B} \subset R^{\text{chartdim}} \mapsto \mathcal{M}
   *   \subset R^{\text{spacedim}}