Fix typo in option help

[nickodell]

There's a typo here, as iterations does not control number of threads.

$ pytest --help

...

run-parallel:
  --parallel-threads=PARALLEL_THREADS
                        Set the number of threads used to execute each test concurrently.
  --iterations=ITERATIONS
                        Set the number of threads used to execute each test concurrently.

[ngoldbaum]

Thanks!