docs: fix Wormhole guardian set command

[tharune]

Summary

• replace the broken raw GitHub directory curl in the Hermes relayer troubleshooting guide
• use the GitHub contents API for Wormhole's current mainnet canonical guardian set files

Verification

• confirmed the old raw.githubusercontent.com directory URL returns 400
• confirmed the GitHub contents API URL returns 200
• ran the replacement command and confirmed it returns the latest canonical set file, currently v5.prototxt
• ran git diff --check