Update tcar-ug book's title.
    
    - Previously, the book had the name "The CentOS Artwork User's Guide".
      The documentation is intended for repository users not artwork
      users.  This update changes the book title from "The CentOS Artwork
      User's Guide" to "The CentOS Artwork Repository User's Guide" to
      reflect that information in this book is intended to repository
      users instead of artwork users.