fix(docs): replace invalid default OpenRouter import with named import

[CryptAm]

Changes

• Replaced the incorrect default import (import OpenRouter from "@openrouter/sdk") with the correct named import (import { OpenRouter } from "@openrouter/sdk") in OVERVIEW.md.

Why

The package does not expose a default export.

This can be verified by following the export chain:

• src/sdk/sdk.ts declares OpenRouter as a named export via export class OpenRouter.
• src/index.ts re-exports it with export * from "./sdk/sdk.js", which only re-exports named exports.
• Neither src/sdk/sdk.ts nor src/index.ts defines or re-exports a default export.

As a result, import OpenRouter from "@openrouter/sdk" is invalid, while import { OpenRouter } from "@openrouter/sdk" matches the package's public API.