R1:f1ca1333ad9c

Fix URI typo in "Configuring File Storage"

Summary: @gschmidt noticed this in IRC.

Test Plan: Verified correct URI is "/file/", eyeballed it.

Reviewers: btrahan, gschmidt

Reviewed By: gschmidt

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1963