[Chronik] Minor fixes in chronik_block.py

Summary: Add a description, fix the class name, use GENESIS_BLOCK_HASH directly

Test Plan: ./test/functional/test_runner.py chronik_block

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13287