fix `torch.futures` docstring examples (#61029)
Summary:
Trying to run the doctests for the complete documentation hangs if it reaches the examples of `torch.futures`. It turns out to be only syntax errors, which are normally just reported. My guess is that `doctest` probably doesn't work well for failures within async stuff.
Anyway, while debugging this, I fixed the syntax.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61029
Reviewed By: mruberry
Differential Revision: D29571923
Pulled By: mrshenli
fbshipit-source-id: bb8112be5302c6ec43151590b438b195a8f30a06
