STYLE: Increase itk::ListSample::PrintSelf consistency

Make `itk::ListSample::PrintSelf` implementation style consistent
following the ITK SWG coding style guideline and the
available ITK macros:
- Print only the class instance variables.
- Use the `os << indent << "{ivarName}: " << m_ivarName << std::endl`
  recipe.

Modules/Numerics/Statistics/include/itkListSample.hxx

@@ -134,8 +134,7 @@ ListSample<TMeasurementVector>::PrintSelf(std::ostream & os, Indent indent) cons
 {
   Superclass::PrintSelf(os, indent);
 
-  os << indent << "Internal Data Container: " << &m_InternalContainer << std::endl;
-  os << indent << "Number of samples: " << this->m_InternalContainer.size() << std::endl;
+  os << indent << "InternalContainer: " << &m_InternalContainer << std::endl;
 }
 } // end of namespace Statistics
 } // end of namespace itk