Minor coding error in create_atoms.cpp

I found a slight error in create_atoms.cpp dealing with inputs for "create_atoms" command.

Lines 129-130:

seed_char = new char [strlen(arg[2])+1];
strcpy(seed_char, arg[2]);

arg[2] should be arg[3], or the number of particles would be taken as prime random seed.

Attached is my modified code. Hope this can be updated into LIGGGHTS git.