Fix typos in signer-requests.md and signers.md

[gasaicrypto]

This PR fixes typos across two documentation files:

Changes

signer-requests.md

• Fixed typo: APP_MNENOMIC → APP_MNEMONIC
• Grammar: "can being writing" → "can begin writing"

signers.md

• Fixed typo: APP_MNENOMIC → APP_MNEMONIC
• Grammar: "signer messages" → "sign messages"

Context

• Documentation-only changes (no code logic affected)
• Typos were causing confusion in code snippets
• Aligns with Ethereum/Hubble terminology

Checklist

• Verified against project glossary
• Changes limited to typos/grammar

---

PR-Codex overview

This PR focuses on correcting typos in the documentation and improving clarity in the warpcast and hubble sections.

Detailed summary

• Corrected APP_MNENOMIC to APP_MNEMONIC in docs/reference/warpcast/signer-requests.md.
• Changed "can being writing messages" to "can begin writing messages" in docs/reference/warpcast/signer-requests.md.
• Updated "signer messages" to "sign messages" in docs/reference/warpcast/signers.md.
• Fixed locaalhost to localhost in docs/hubble/migrating.md.
• Changed "writing dia" to "writing data" in docs/hubble/migrating.md.
• Added a newline at the end of docs/hubble/migrating.md.

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