Fix commented out DB_URL strings for MySQL and PostgreSQL in sample config
    
    The formatting of the 'DB_URL' strings for MySQL and PostgreSQL were slightly
    wrong, leading to confusion when switching from SQLite to either database in
    the configuration.
    
    Merges https://pagure.io/pagure/pull-request/3276
    
    Signed-off-by: Neal Gompa <ngompa13@gmail.com></ngompa13@gmail.com>
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>