Tell the user how to remap 'Open Prompt Palette' keyboard shortcut

We discussed offline that it may be beneficial to tell the user to go to `chrome://extensions/shortcuts` in case the keyboard shortcut doesn't work, or they want to set their own custom key mapping

_Originally posted by @mickmister in https://github.com/aelishRollo/prompt-palette/pull/34#discussion_r1955373288_

This change should happen both in the P.P. help menu and in the readme