Rollup merge of rust-lang#87422 - mgeier:doc-strip-feature-attribute,…

… r=LeSeulArtichaut

DOC: remove unnecessary feature crate attribute from example code

I'm not sure whether I fully understand the stabilization process (I most likely don't), but I think this attribute isn't necessary here, right?

This was recently stabilized in rust-lang#86344.

library/core/src/mem/maybe_uninit.rs

@@ -461,7 +461,6 @@ impl<T> MaybeUninit<T> {
     /// With `write`, we can avoid the need to write through a raw pointer:
     ///
     /// ```rust
-    /// #![feature(maybe_uninit_extra)]
     /// use core::pin::Pin;
     /// use core::mem::MaybeUninit;
     ///