[dev] fix arcanist config

Summary: arc patch wasn't working for me because the phabricator uri in .arcconfig wasn't updated. this config change fixed things.

Test Plan: was able to run arc patch successfully

Reviewers: palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4222