Update centos-art.sh script's tuneup->xhtml module.
    
    - Previously, when building the anchor reference, it was used the
      entire heading line. So, each time you ran tuneup module, the anchor
      references were updated with a different hash value because the
      previous hash was also considered to build the new one. This make
      external references to pages transformed this way to fail. This
      commit uses only the heading content as reference and excludes
      everything else in the heading line.  This makes external references
      pointing headings inside pages transformed through tuneup->xhtml
      module to always point the same heading location (considering the
      heading content has not changed since the last execution of
      tuneup-xhmlt module).