Update combination_sum.py

Author: MaximSmolskiy

backtracking/combination_sum.py

@@ -47,7 +47,7 @@ def combination_sum(candidates: list, target: int) -> list:
     >>> combination_sum([-8, 2.3, 0], 1)
     Traceback (most recent call last):
         ...
-    ValueError: All elements in candidates must be non-negative.
+    ValueError: All elements in candidates must be non-negative
     >>> combination_sum([], 1)
     Traceback (most recent call last):
         ...
@@ -57,7 +57,7 @@ def combination_sum(candidates: list, target: int) -> list:
         raise ValueError("Candidates list should not be empty")
 
     if any(x < 0 for x in candidates):
-        raise ValueError("All elements in candidates must be non-negative.")
+        raise ValueError("All elements in candidates must be non-negative")
 
     path = []  # type: list[int]
     answer = []  # type: list[int]