Update build-on-casper.md

Proposing to update additional possible Free RPC endpoint powered by All That Node.

Proposing to update additional possible public RPC endpoint powered by All That Node.

Closes #

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[ ] Docs are successfully building - yarn install && yarn run build.
[ ] For new internal links I used relative file paths (with .md extension) - e.g. ../../faq/faq-general.md - instead of introducing absolute file path, or relative/absolute URL.
[ ] All external links have been verified with yarn run check:externals.
[ ] My changes follow the Casper docs style guidelines.
[ ] All technical procedures have been tested (if you want help with this, mention it in Reviewers).
[ ] If structural changes are introduced (not just content changes), cross-broswer testing has been completed.

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casper-network / docs