Fix deprecation warning for compare_and_swap in atomics.md

[Unconnectable]

Description:

The current example code triggers the following deprecation warning in recent Rust versions:

warning: use of deprecated method `std::sync::atomic::AtomicBool::compare_and_swap`: Use `compare_exchange` or `compare_exchange_weak` instead
  --> src/main.rs:11:16
   |
11 |     while lock.compare_and_swap(false, true, Ordering::Acquire) {}
   |                ^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(deprecated)]` on by default

Changes:

This PR replaces the deprecated compare_and_swap with compare_exchange:

// Before
while lock.compare_and_swap(false, true, Ordering::Acquire) { }

// After
while lock.compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed).is_err() { }

Technical Note on compare_exchange vs compare_exchange_weak: While compare_exchange_weak is often preferred in loop scenarios for potential performance gains on certain architectures, I have opted for the standard compare_exchange here. This maintains exact semantic parity with the original example and avoids the need for additional explanations regarding "spurious failures," keeping the focus on memory ordering as intended in the nomicon.

The compare_and_swap method is deprecated in favor of compare_exchange. This update uses Ordering::Acquire for success and Ordering::Relaxed for failure, which is appropriate for a spinlock acquire loop.

[JohnTitor]

Good catch!