Fix missing imports from Node SDK usage

[wobsoriano]

In the Node SDK example on the Clerk docs site, there are sections where resources are destructured from the clerkClient function and imported directly. These methods are not exported, resulting in an error.

• https://clerk.com/docs/references/nodejs/overview#manual-authentication
• https://clerk.com/docs/references/nodejs/available-methods#instantiate-a-default-clerk-client-instance

This change updates those sections to just import clerkClient and access the resource from it.

Before:

import { sessions } from '@clerk/clerk-sdk-node';

// more code

const session = await sessions.verifySession(sessionId, clientToken);

After:

import { clerkClient } from '@clerk/clerk-sdk-node';

// more code

const session = await clerkClient.sessions.verifySession(sessionId, clientToken);