[doc][handbook] Improve section on VTK.

doc/handbook/6_inputoutput.tex

@@ -38,8 +38,8 @@ Furthermore, petrophysical properties can be defined (for each cell), by using e
 \Dumux supports the Eclipse Grid Format by using the \texttt{opm-grid} module (see (\url{https://opm-project.org}).
 An example can be found in \texttt{dumux/test\allowbreak/porousmediumflow/2p/implicit/cornerpoint}.
 
-\subsubsection{VTK file format}
-VTK format uses ASCII and XML format. It is mostly used by \Dumux for output purposes and can be visualized by programs such as Paraview. Using VTK files to input grid and parameter data is also possible.
+\subsubsection{VTK File Format}
+VTK format uses ASCII or XML format. It is mostly used by \Dumux for output purposes and can be visualized by programs such as Paraview, ViIt or Tecplot. Using VTK files to input grid and parameter data is also possible.
 An example can be found in \texttt{dumux/test\allowbreak/io/gridmanager}.
 
 \subsubsection{Other Grid Formats}