fix(docs): remove .mdx extension from signInWithEthereum link in wallet_connect

[Kewe63]

Summary

Closes #1311

The internal link at docs/base-account/reference/core/provider-rpc-methods/wallet_connect.mdx:193 included a .mdx file extension which caused the rendered Mintlify docs site to return 404 when clicked. Mintlify resolves bare paths to their target pages automatically — the extension is not required and trips the URL.

Removed the trailing .mdx from the signInWithEthereum link target.

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Verification

• grep -n signInWithEthereum on the file shows only line 193 holds an /.../signInWithEthereum.mdx style link — the rest are code-block references, ParamField definitions, or JSON examples that don't carry the .mdx extension.
• Reproduced from the report: the live docs site 404s on …/signInWithEthereum.mdx and returns 200 on …/signInWithEthereum (no extension).
• Mintlify docs.site/build/config behavior: extensionless routes are the canonical form for internal cross-references.

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Checklist

• Documentation-only change (no code, no config, no images)
• Single-line, scoped fix — no other surfaces touched
• git diff --stat shows 1 file, +1/-1
• Pre-merge: verify the link resolves on https://docs.base.org/base-account/reference/core/provider-rpc-methods/wallet_connect (Mintlify preview build)

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Risk & Impact

None. Single-line documentation link fix. No code, config, or behavior changes.

Type: 📝 Documentation
Closes: #1311

[cb-heimdall]

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