doc: fix BrowserManager import in serverless example

[yannliao]

updates BrowserManager import issue in serverless example.

[vercel]

@yannliao is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[ctate]

Good catch! The fix is correct, but there's a second instance on line 557 that also needs updating:

// Line 557 - Programmatic API section
import { BrowserManager } from 'agent-browser';

Could you update that one as well? Both examples should use 'agent-browser/dist/browser'.