Mistake in AtomicBool spinlock example

The current example of a spinlock was not correct. The lock is actually acquired
when old == result. So we only need to deschedule when this is not the case.
auto merge of #15575 : mvdnes/rust/spinlock_error, r=alexcrichton

The current example of a spinlock was not correct. The lock is actually acquired
when `old == result`. So we only need to deschedule when this is not the case.