[async.use.future.members] defined in terms of invalid completion signature

[jwakely]

[async.use.future.members] says

Let Args... by the completion signature of H

I think "by" should be "be"

More importantly, a completion signature should be a call signature like R(Args...), not just a pack expansion.

I haven't been able to think of a fix for the wording, because if I understand correctly the choice of R is arbitrary here, it isn't actually used because the return_type only depends on result_of_t<_Func(decay_t<_Args>...)> not R.

Maybe something like ...

Let R(Args...) by the completion signature of an asynchronous operation using H

[chriskohlhoff]

Applied suggested fix in c231e29881e383f1bce93d3c63170fd551e3adff.