DOC: Fix typos and improve Doxygen related pages documentation

Fix typos in Doxygen related pages various documentation modules.

Take advantage of the commit to improve the `Geometry` related pages
documentation.

Documentation/Doxygen/Geometry.dox

@@ -1,13 +1,14 @@
 /**
   \page GeometryPage Geometry Concepts
-
-  Insight provide basic classe to represent geometrical concepts, alghought
-  the aim of the toolkit is not computational geometry, some of these elements
-  are necessary for common procedures in image processing.
 
-  This document present the different geometric concepts used in the toolkit,
+  ITK provides basic classes to represent geometrical concepts. Despite
+  not being a computational geometry modelling toolkit, ITK offers a set
+  of such elements necessary for common procedures in image processing.
+
+  This document presents the different geometric concepts used in the toolkit,
   as well as their relationships and how they can effectively be used.
-
-  At the beggining there was the Point
+
+  The toolkit supports representing meshes of different nature and points
+  through the itk::Mesh, itk::QuadEdgeMesh, and itk::Point classes.
 
  */

Documentation/Doxygen/Registration.dox

@@ -46,7 +46,7 @@ In order to determine the registration method appropriated for a particular prob
 
 \subsection TargetReference Target and Reference Objects
 
-Currently the Target an Reference objects can be of type itk::Image and itk::PointSet. Methods have been instantiated for a variety of <em> Image to Image </em> and <em> PointSet to Image </em> registration cases.
+Currently the Target and Reference objects can be of type itk::Image and itk::PointSet. Methods have been instantiated for a variety of <em> Image to Image </em> and <em> PointSet to Image </em> registration cases.