Remove errant --testnet argument in chainparams README

Summary:
This argument was leftover from an old design. It's no longer necessary/relevant and fails
when you copy-paste this command.

Test Plan:

bitcoind --testnet
make_chainparams --testnet -a 127.0.0.1:18332 > chainparams_test.txt  # fails
make_chainparams -a 127.0.0.1:18332 > chainparams_test.txt  # successful

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4650