Update centos-art.sh script copyright holder.
    
    - Previously, we were using The CentOS Project as copyright holder for
      centos-art.sh script. The CentOS Project main goal is The CentOS
      Distribution not the centos-art.sh script. The centos-art.sh script
      exists to automate frequent tasks inside The CentOS Artwork
      Repository which is the main goal of The CentOS Artwork SIG. In this
      sake, The CentOS Artwork SIG is closer than The CentOS Project and
      should be used, instead. This update changes centos-art.sh scripts
      and set The CentOS Artwork SIG as copyright holder.