Fix double menu item separator in "Help" section of app menu (#8351)

I've modified the code at `crates/zed/src/app_menus.rs` as it looks like
there was a typo and instead of one
`MenuItem::separator()` there were two.

It would be something like this:

<img width="1512" alt="Captura de pantalla 2024-02-25 a las 16 12 10"
src="https://github.com/zed-industries/zed/assets/93369643/d35d5b57-247c-4a1f-b65a-a1b68cd302b9">