rust: sync: atomic: Add example for Atomic::get_mut()

rust: sync: atomic: Add example for Atomic::get_mut()

Add an example for Atomic::get_mut(). No functional change.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260128123313.3850604-1-tomo@aliasing.net
Link: https://patch.msgid.link/20260303201701.12204-3-boqun@kernel.org