Grammar fix in MovePicker::next_move

Thanks to Lyudmil Antonov and Michel Van den Bergh
for spotting this and suggesting the fix.

No functional change.
Fix null reduction formula

Depth is already dependent on the actual value
of ONE_PLY, in particular can be expressed like:

Depth = n * ONE_PLY

And because formula is used to calculate R that is
also dependent on the value of ONE_PLY and can be
expressed like:

R = x * ONE_PLY

We don't want to divide depth by a 'ply' value but
directly by an integer number.

Spotted by sf-x

No functional change.
Merge remote-tracking branch 'upstream/master'