Corrected description of the "//" operator

The operator was previously described to truncate the the value when it in fact floors the result of the division (which can be seen in https://github.com/fabpot/Twig/blob/master/lib/Twig/Node/Expression/Binary/FloorDiv.php#L20).
By the old description one would think that ``{{ -20 // 7 }}`` would yield -2 (as in ``(int)(-20/7)``) when it actually returns -3 (as in ``floor(-20/7)``). Floor is not truncate (cast to int).