Cleanup the Put function in the FIFO cache

[iFrostizz]

As correctly pointed out by @StephenButtolph , and quoting his message about the Put function:

Total aside but Put  would probably be cleaner if it was:
func (f *FIFO[K, V]) Put(key K, val V) bool {
	f.l.Lock()
	defer f.l.Unlock()

	_, exists := f.m[key]
	if !exists {
		f.buffer.Push(key) // Insert will remove the oldest [K] if we are at the [limit]
	}
	f.m[key] = val
	return exists
}

The comment should probably be fixed as well:
// Insert will remove the oldest [K] if we are at the [limit]
Should be:
// Push removes the oldest [K] if we are at the [limit]