Rewrite extensions page info message

[luceos]

Admin extensions page mentions:

In the meantime, if you manage to get your hands on a new extension, simply drop it in your forum's extensions directory.

This leads to a lot of questions of why it's not working.

You have to use composer in ssh/bash to get an extension working properly. Composer automatically generates a "map" of all code, without using composer the code in the extension will not be available to Flarum.

[franzliedke]

This has already been fixed a few days ago when @dcsjapan extracted these strings to make them translateable.

[luceos]

Cool, couldn't find that change anywhere.