[docs] Update Flipper plugins instructions to navigate to Plugin Manager

Summary:
On the dev environment docs for Flipper plugins, it erroneously states:

There is no Manage Plugins in the View menu.

Instead, the Plugin Manager can be found here:

Updated the docs to reflect this change.

Linear issue with full context here.

Test Plan: N/A

Reviewers: ashoat, varun

Reviewed By: ashoat, varun

Subscribers: palys-swm, Adrian, atul

Differential Revision: https://phab.comm.dev/D4256