doc: Fix -dustrelayfee= argument docs grammar
Summary:
One character fix in documentation of -dustrelayfee parameter
Backport of Core PR15877
Test Plan: ninja && src/bitcoind -help
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7974