Update Phalcon_Db_Adapter_Pdo_Mysql.rst

Code in *fetchOne* change from
$robot = $connection->fecthOne("SELECT * FROM robots");
$robot = $connection->fecthOne("SELECT * FROM robots", Phalcon\Db::FETCH_ASSOC);
to
$robot = $connection->fetchOne("SELECT * FROM robots");
$robot = $connection->fetchOne("SELECT * FROM robots", Phalcon\Db::FETCH_ASSOC);
Merge pull request #154 from relort/patch-11

Update Phalcon_Db_Adapter_Pdo_Mysql.rst