Fix a typo in documentation
Fix a typo in documentation

Summary: `fpipe()` is not a real thing, this should be `fwrite()`.

Test Plan: Reading.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14921

Symplicity/libphutil,Symplicity/libphutil,Symplicity/libphutil