Rollup merge of #104332 - Elarcis:maybe_uninit_doc_fix, r=m-ou-se

Fixed some `_i32` notation in `maybe_uninit`’s doc

This PR just changed two lines in the documentation for `MaybeUninit`:

```rs
let val = 0x12345678i32;
```
was changed to:
```rs
let val = 0x12345678_i32;
```
in two doctests, making the values a tad easier to read.

It does not seem like there are other literals needing this change in the file.

library/core/src/mem/maybe_uninit.rs

@@ -1172,7 +1172,7 @@ impl<T> MaybeUninit<T> {
  /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
  /// use std::mem::MaybeUninit;
  ///
- /// let val = 0x12345678i32;
+ /// let val = 0x12345678_i32;
  /// let uninit = MaybeUninit::new(val);
  /// let uninit_bytes = uninit.as_bytes();
  /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(uninit_bytes) };
@@ -1198,7 +1198,7 @@ impl<T> MaybeUninit<T> {
  /// #![feature(maybe_uninit_as_bytes)]
  /// use std::mem::MaybeUninit;
  ///
- /// let val = 0x12345678i32;
+ /// let val = 0x12345678_i32;
  /// let mut uninit = MaybeUninit::new(val);
  /// let uninit_bytes = uninit.as_bytes_mut();
  /// if cfg!(target_endian = "little") {