[Nix Docs] Edit Phabricator account section for continuity

Summary:
Modify wording to fit better in the new context.

Depends on D6725

Test Plan:
N/A

Rendered: https://github.com/CommE2E/comm/blob/jonringer/nix-docs-supercede/docs/nix_dev_env.md

Reviewers: ashoat, varun

Reviewed By: ashoat, varun

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6726