fix: correct RPC env var name in Foundry tutorial

[jrootcache]

Problem

In the Foundry deployment tutorial (src/pages/build/tutorials/deploying-a-smart-contract/foundry.mdx), the foundry.toml configuration references an environment variable ${INKSEPOLIA_RPC_URL} for the Ink Sepolia RPC endpoint. However, the accompanying .env file example in the same tutorial defines RPC_URL, not INKSEPOLIA_RPC_URL.

This mismatch causes the RPC endpoint to resolve to an empty string, breaking deployment to Ink Sepolia for users following the tutorial.

Fix

Changed the foundry.toml rpc_endpoints configuration from ${INKSEPOLIA_RPC_URL} to ${RPC_URL} to match the variable name used in the .env example.

Why this is safe

• Single-line change: only the environment variable name in the Foundry config.
• The new value ${RPC_URL} exactly matches the variable defined in the tutorial's own .env example (line 122).
• No logic changes; only fixes a documentation inconsistency that would cause runtime failures.

[jrootcache]

@przemekink @sergio-inkfnd @devknight-ink

Could someone review this when you have a moment?

This fixes a broken RPC config in the Foundry tutorial — the foundry.toml references ${INKSEPOLIA_RPC_URL} but the .env example defines RPC_URL. Anyone following the guide would hit a silent RPC failure on deploy. One-line fix aligns them.

Thanks!