Update README.md

Author: kevinlu1248

Lesson 1/README.md

@@ -2,7 +2,7 @@
 1. [introduction.md](https://github.com/kevinlu1248/ccc_python_lessons/tree/master/Lesson%201/introduction.md): (10 minutes) Go over what they want to do, Github Student Pack and environment such as Pycharm as they eat
 2. [review.py](https://github.com/kevinlu1248/ccc_python_lessons/tree/master/Lesson%201/review.py): (5 minutes) ask what they want to do and review basic Python (string formatting, conditionals, loops, and functions)
 3. [zip.py](https://github.com/kevinlu1248/ccc_python_lessons/tree/master/Lesson%201/zip.py): (10 minutes) go over a commonly used function in CCC: zip; also there is an activity
-4. Try doing https://dmoj.ca/problems/ccc18j4 using zip!
+4. Try doing https://dmoj.ca/problems/ccc18s2 using zip!
 5. Bonus: try https://dmoj.ca/problem/ccc16j2 with zip as well
 
 **Note that each of the Python file's teaching materials can be found in the Lesson 1 repository at:**