[Nix Docs] Update terminal powerline font instructions

Summary:
Sometime between when the docs were written and macOS Ventura, the way
in which someone sets the terminal font has changed. Update the instructions to
the most recent workflow.

https://linear.app/comm/issue/ENG-2360

Test Plan:

# open terminal application

# reset powerline preference
rm -f "$HOME/Library/Caches/app.comm/enable-powerline"

nix develop

# Answer y to powerline prompt
# See new message, and ensure that it's the correct location

Reviewers: ashoat, varun, O1 Nix, atul

Reviewed By: ashoat, O1 Nix, atul

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D5742