Fix up release note on parked chain RPC

Summary: Missed adding a note on the new RPC commands

Test Plan: Read it

Reviewers: deadalnix, schancel, Fabien, #bitcoin_abc

Reviewed By: deadalnix, schancel, Fabien, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2020