formatting typos in Function.md
formatting typos in Function.md

Summary: Fixes two tiny formatting issues with the folly::Function documentation.

Reviewed By: yfeldblum

Differential Revision: D7856507

fbshipit-source-id: dc82256a35f6c2b1eb66a868f419f2c800f1ddc5

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