Description comment in pt_join.c

[lmartak]

I'd like to point out that commentary description of this file is probably outdated/incorrect/misleading. https://github.com/drahosp/PaPP/blob/master/examples/pthreads/pt_join1.c

Specifically following part seems to be different:

Each thread draws one million random numbers between 0 and 1 and adds these together in a variable sum.

since threads only do increment of shared variable count.