Set guard-n-primary-dir-guards-to-use=2

As per Prop#344 Section 1.3.1 (https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/proposals/344-protocol-info-leaks.txt#L657), the use of 3 directory guards is a highly unique fingerprint that can be used to track Tor users as they move from network to network.

Lowering this param to 2 improves the situation somewhat.