raster3dintro manual: cosmetics

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@61349 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster3d/raster3dintro.html

@@ -1,5 +1,7 @@
 <!-- meta page description: 3D raster data (voxel) processing in GRASS GIS -->
 <!-- meta page index: raster3D -->
+<h3>3D Raster maps (voxels) in general</h3>
+
 GRASS GIS is one of the few GIS software packages with volume data support.
 Here data are stored as a 3D raster with a unit volume called a voxel
 (volume pixel). Voxels are designed to support representations of