[Nix docs] Correct port for web workflow

Summary:
Update docs to point to correct url. Since the nix worklfow isn't
using a service like apache to pass through port 80 traffic, using the default
http port won't work.

Test Plan: N/A

Reviewers: ashoat, varun, atul, abosh

Reviewed By: ashoat, atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D5275