Update .arcconfig phabricator URI to the new syntax

Summary:
This is set according to
https://secure.phabricator.com/book/phabricator/article/arcanist_new_project/,
the previous syntax being incompatible with the latest arcanist
versions. The previous one is kept for compatibility.

Test Plan:

arc diff
arc land --preview

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6704