Fix vtkAxisActor2D documentation error

This commit fixes the documentation of the default value of vtkAxisActor2D's
SizeFontRelativeToAxis property. By default, fonts are resized relative to the
viewport.

Rendering/Annotation/vtkAxisActor2D.h

@@ -291,7 +291,7 @@ public:
 
   // Description:
   // Specify whether to size the fonts relative to the viewport or relative to
-  // length of the axis. By default, fonts are resized relative to the axis.
+  // length of the axis. By default, fonts are resized relative to the viewport.
   vtkSetMacro(SizeFontRelativeToAxis,int);
   vtkGetMacro(SizeFontRelativeToAxis,int);
   vtkBooleanMacro(SizeFontRelativeToAxis,int);