trait.MiscExt is missing

[Bilal Elmoussaoui]

gtk::Label implements MiscExt but clicking on it returns a 404 error
https://gtk-rs.org/docs/gtk/trait.MiscExt.html

I think the documentation should reflect that & remove it as it's not used anywhere in gtk-rs/gtk

Here is where the MiscExt trait is mentioned
https://github.com/gtk-rs/docs/search?q=MiscExt&unscoped_q=MiscExt