Remove references to `Futures()` from the documentation
Remove references to `Futures()` from the documentation

Summary: These were missed in D11081.

Test Plan: Eyeball it.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: richardvanvelzen, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11269

julkiewicz/libphutil,julkiewicz/libphutil,julkiewicz/libphutil