fix mismatching arg names in avalanche RPC commands

Summary:
This makes the arg names identical in the RPC function and in the corresponding registered command.

In the next commit, an assertion will be added to avoid any future similar regression.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10741