Fix flag description for Browser Session Management

[manuelgruber]

This pull request updates the documentation for managing named browser sessions in Playwright CLI. The main change is clarifying the correct flag to use for isolating browser contexts.

Documentation update:

• Updated the session management documentation in session-management.md to indicate that the -s flag (instead of -b) should be used to isolate browser contexts.

[Copilot]

Pull request overview

This PR corrects the Playwright CLI session management documentation to reference the proper flag for isolating named browser sessions (browser contexts).

Changes:

• Updated the session isolation guidance from using -b to using -s.

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[manuelgruber]

Closes #243

[Skn0tt]

Hi! We renamed this a couple days ago, see microsoft/playwright#39156. So I think -b is right, maybe you're using a slightly old version of the CLI?

[yury-s]

You are right about the issue, the file used to create the kill via playwright-cli install lives upstream thought, I'm updating it there microsoft/playwright#39200.