[lammps-users] manual / directory of executable

In docs in "2.5 Running LAMMPS" we have:

This is an MPI issue, not a LAMMPS issue. Some MPIs change
the working dir to where the executable is when you do an mpirun.
If it works without copying the exe to the working dir on your box,
then great.

I've amended the doc file to clarify this.