Make examples for AtomicInt refer to AtomicInt

The examples for fetch_or, fetch_and and fetch_xor for
std::sync::atomic::AtomicInt used AtomicUint instead of AtomicInt.
auto merge of #17766 : pminten/rust/atomic-int-in-examples, r=alexcrichton

The examples for fetch_or, fetch_and and fetch_xor for
std::sync::atomic::AtomicInt used AtomicUint instead of AtomicInt.