added doctest in all_combinations.py

Author: Siddhant231xyz

backtracking/all_combinations.py

@@ -17,8 +17,10 @@ def combination_lists(n: int, k: int) -> list[list[int]]:
     >>> combination_lists(n=4, k=2)
     [[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]
     >>> combination_lists(n=5, k=3)
-    [[1, 2, 3], [1, 2, 4], [1, 2, 5], [1, 3, 4], [1, 3, 5], [1, 4, 5], [2, 3, 4], [2, 3, 5], [2, 4, 5], [3, 4, 5]]
-    >>> combination_lists(n=0, k=0)
+    [[1, 2, 3], [1, 2, 4], [1, 2, 5],
+     [1, 3, 4], [1, 3, 5], [1, 4, 5],
+     [2, 3, 4], [2, 3, 5], [2, 4, 5],
+     [3, 4, 5]]
     []
     >>> combination_lists(n=1, k=1)
     [[1]]