Update mpi-weakScaling.sh

Fixed error in -np 4 case so that it actually is weak scaling.

examples/mpi-weakScaling.sh

@@ -3,6 +3,6 @@
 # Simple weak scaling study with eam potential and 32000 atoms per task
 mpirun -np 1  ../bin/CoMD-mpi -e -i 1 -j 1 -k 1 -x 20 -y 20 -z 20
 mpirun -np 2  ../bin/CoMD-mpi -e -i 2 -j 1 -k 1 -x 40 -y 20 -z 20
-mpirun -np 4  ../bin/CoMD-mpi -e -i 2 -j 2 -k 1 -x 40 -y 40 -z 40
+mpirun -np 4  ../bin/CoMD-mpi -e -i 2 -j 2 -k 1 -x 40 -y 40 -z 20
 mpirun -np 8  ../bin/CoMD-mpi -e -i 2 -j 2 -k 2 -x 40 -y 40 -z 40
 mpirun -np 16 ../bin/CoMD-mpi -e -i 4 -j 2 -k 2 -x 80 -y 40 -z 40