[chronik] Electrum: fix txid vs tx_hash the parameter name

Summary: The protocol uses tx_hash instead of txid so we use that to remain compatible with clients using named arguments.

Test Plan:

./test/functional/test_runner.py chronik_electrum_*

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18191