fix warnings

ethcore/src/trace/flat.rs

@@ -14,6 +14,8 @@
 // You should have received a copy of the GNU General Public License
 // along with Parity.  If not, see <http://www.gnu.org/licenses/>.
 
+//! Flat trace module
+
 use trace::BlockTraces;
 use super::trace::{Trace, Action, Res};