Typo fix and better paraview explanation

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@45444 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.to.rast3elev/r.to.rast3elev.html

@@ -43,7 +43,7 @@ r.to.rast3elev --o in=soils elev=elevation.10m out=volev_l -l
 
 r.to.rast3elev --o in=soils elev=elevation.10m out=volev_u -u
 
-# And Example with multiple elevation maps.
+# Example with multiple elevation maps.
 ## first we need three support maps
 
 r.mapcalc --o expression="one = 1" 
@@ -59,8 +59,16 @@ r.mapcalc --o expression="elev_bottom = elevation.10m - 1000"
 
 r.to.rast3elev --o -l input=one,two,three elevation=elevation.10m,elev_mid,elev_bottom output=threelayer
 
-## Export the map for visualization with paraview
-r3.out.vtk --o input=threelayer output=/tmp/threelayer.vtk
+## Export the map for visualization with paraview (http://www.paraview.org)
+## By default the null value is -9999.99, we adjust it to 0.0 for 
+## better visualization 
+r3.out.vtk --o null=0.0 input=threelayer output=/tmp/threelayer.vtk
+
+# Start paraview 
+
+paraview --data=/tmp/threelayer.vtk
+
+# First you need to choose the surface representation style and then color by "threelayer" in paraview . 
 
 </pre></div>