Remove maybe_uninit_extra feature from Vec docs

In `Vec`, two doc tests are using `MaybeUninit::write` , stabilized in 1.55. This makes docs' usage of `maybe_uninit_extra` feature unnecessary.

Author: Shadlock0133

Diff for: library/alloc/src/vec/mod.rs

@@ -2043,7 +2043,7 @@ impl<T, A: Allocator> Vec<T, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(vec_spare_capacity, maybe_uninit_extra)]
+    /// #![feature(vec_spare_capacity)]
     ///
     /// // Allocate vector big enough for 10 elements.
     /// let mut v = Vec::with_capacity(10);
@@ -2102,7 +2102,7 @@ impl<T, A: Allocator> Vec<T, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(vec_split_at_spare, maybe_uninit_extra)]
+    /// #![feature(vec_split_at_spare)]
     ///
     /// let mut v = vec![1, 1, 2];
     ///