Minor change for consistent capitalization throughout the README.md

[mmyyrroonn]

This PR:

It make a consistent capitalization throughout the document for the Light Client smart contract

This PR does not:

It does not include any code change.

Key places to review:

It follows the pattern which appears the first time https://github.com/EspressoSystems/espresso-sequencer/blob/7184e3cce876866ef6a3ae7e59dd1fb7ff957d17/README.md?plain=1#L41