gfortran documentation fix: replace getpid() with THIS_IMAGE() in init_random_seed()

[FX]

> I probably should have made clear that the motivation was that the Cray compiler doesn't support the getpid() extension so my primary question is how would you suggest  to replace this extension in the interest of portability?  Any suggestions in that direction would be greatly appreciated.  

On the other hand, portability to other compilers is not the primary concern in our own documentation.

FX