[mock-chronik-client] minor README formatting updates

Summary: Removing unintended tabs in README

Test Plan: Plonk readme into https://jbt.github.io/markdown-editor/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14449