Update FAQ text related to Developer Tools split

[Hans5958]

The string "Scratch 3 Developer Tools: This browser extension is a copy of the Developer tools addon. You should uninstall the Developer Tools browser extension and turn on the addon instead." is incorrect, the addon has been split.

Originally posted by @rimopa in #235 (comment)

[Bashamega]

Dear Mr. Han,
I would like to work on this issue. Just tell me the new text.

[Hans5958]

Welcome! I'd suggest you make yourself familiar with the extension before proceeding.

Unfortunately, I haven't got one in mind (I would do it if I had, I think). If you have an idea, you can work on it by yourself, and get it reviewed.

PS. Oh, wow. Didn't realised Hacktoberfest is on action now.