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">

crates/zed/src/app_menus.rs

@@ -161,7 +161,6 @@ pub fn app_menus() -> Vec<Menu<'static>> {
                     workspace::ToggleGraphicsProfiler,
                 ),
                 MenuItem::separator(),
-                MenuItem::separator(),
                 MenuItem::action(
                     "Documentation",
                     crate::OpenBrowser {