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

## Bug

`docs/base-account/reference/core/provider-rpc-methods/wallet_connect.mdx` line 193 links to `/base-account/reference/core/capabilities/signInWithEthereum.mdx` with a `.mdx` extension which returns 404.

**Steps to reproduce:**
1. Visit https://docs.base.org/base-account/reference/core/provider-rpc-methods/wallet_connect
2. Click the `signInWithEthereum` link
3. Gets 404

**Root cause:**
Link incorrectly includes `.mdx` extension:
- ❌ `/base-account/reference/core/capabilities/signInWithEthereum.mdx` → 404
- ✅ `/base-account/reference/core/capabilities/signInWithEthereum` → 200

**Affected file:**
`docs/base-account/reference/core/provider-rpc-methods/wallet_connect.mdx` line 193