Fix error messages for input validation

Author: cclauss

sorts/topological_sort.py

@@ -32,12 +32,12 @@ def topological_sort(start: str, visited: list[str], sort: list[str]) -> list[st
     >>> topological_sort("a", "b", "c")
     Traceback (most recent call last):
         ...
-    ValueError: visited must be a list"
+    ValueError: visited must be a list
 
     >>> topological_sort("a", [], "c")
     Traceback (most recent call last):
         ...
-    ValueError: sort must be a list"
+    ValueError: sort must be a list
     """
     if not isinstance(visited, list):
         raise ValueError("visited must be a list")