Use constants in seeder help text

Summary:
These numbers are hard coded into the help text. Using the constants directly makes
the help text more future proof.

Test Plan:

./bitcoin-seeder -h

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10215