fix(docs): replace --etherscan-api-key with Blockscout verifier flags in Foundry guide

[schoolkamsergj]

Summary

Fixes the incorrect contract verification command in the Foundry deployment guide raised in #609.

The foundry.mdx guide used --etherscan-api-key flag when targeting Ink Sepolia network, which uses Blockscout — not Etherscan. This caused forge verify-contract to fail with an "unsupported chain" error.

Change

Replaced the incorrect Etherscan flag with the proper Blockscout verifier flags.

Before:

forge verify-contract <DEPLOYED_CONTRACT_ADDRESS> src/InkContract.sol:InkContract \
    --chain-id 763373 \
    --etherscan-api-key $BLOCKSCOUT_API_KEY

After:

forge verify-contract <DEPLOYED_CONTRACT_ADDRESS> src/InkContract.sol:InkContract \
    --chain-id 763373 \
    --verifier blockscout \
    --verifier-url https://explorer-sepolia.inkonchain.com/api \
    --verifier-api-key $BLOCKSCOUT_API_KEY

Also updated the section heading from "on Etherscan" to "on Blockscout" for consistency.

Why

• Etherscan and Blockscout use different APIs and are not interchangeable
• Ink Sepolia is verified through Blockscout at https://explorer-sepolia.inkonchain.com/api
• Without this fix, forge verify-contract fails for all Ink Sepolia deployments

Closes #609