pf.conf.5: fix ≤

pf.conf expects <=, not ≤ (and the same applies to ≥ and >=).
Make sure the man page reflects this.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 85c333a947e950d9267517afd1d9c30a655b7bfb)