Update build-on-casper.md

[yunseonna]

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

What does this PR fix/introduce?

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

Closes #

Additional context

// Add further detail on the current situation, or what the PR seeks to address.
// Include loom, screenshots, or gifs(record, compress) if helpful.

Checklist

(Delete any that aren't relevant)

• 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.

Reviewers

// Tag whoever needs to review. If you're not sure, leave blank.

[ipopescu]

@yunseonna please add a link to the faucet https://testnet.cspr.live/tools/faucet.