docs: typo fix another

Co-authored-by: Steve Piercy <[email protected]>

src/colander/__init__.py

@@ -107,7 +107,7 @@ def __init__(self, node, msg=None, value=None):
  self.children = []
 
  def messages(self):
- """Return an iterable of error messages for this exception
+ """Return an iterable of error messages for this exception.
 
  Uses the ``msg`` attribute of this error node.