fix: use the correct key to search the value

[hi-jennie]

Because it uses the key to search for the entry, it always searches for the first entry’s key instead of the specified k.
If you want me to add an unit test, I'm glad to add it.

public static void main(String[] args) {
        ULLMap<String, Integer> map = new ULLMap<>();
        map.put("Jennie", 24);
        map.put("Rustin", 28);
        System.out.println(map.get("Jennie"));
}

Before I correct this problem, it always prints null. After correction, it prints out 24.