Update parallel.h

[lanvent]

No description provided.

[tomtseng]

@ldhulipala are you familiar enough with OpenMP to comment on this? This change looks correct -- without it, I think left() and right() run sequentially. But maybe it's better to have --- instead of having the current thread spawn a task for left() + spawn a task for right() --- the current thread spawn a task only for left() and to run right() itself before waiting. The other logic in pardo() for OpenMP also looks suspicious to me. If someone runs a par_do in a parallel_for, does the bool in_par_do logic get messed up? And it seems bad to have nested pardos not taskwait since our assumption when programming is that after a pardo, both left() and right() have completed.