Add tag_t to Futures.md
Add tag_t to Futures.md

Summary: Update futures documentation for consistency with tag type changes.

Reviewed By: yfeldblum

Differential Revision: D13975003

fbshipit-source-id: 7015ffb8bd2b306e2dd31f6e2eb9e45b2618fa5f

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly