fix a typo in stdlib/3/typing.py

Author: lunixbochs

stdlib/3/typing.pyi

@@ -5,7 +5,7 @@ from abc import abstractmethod, ABCMeta
 from types import CodeType, FrameType, TracebackType
 import collections  # Needed by aliases like DefaultDict, see mypy issue 2986
 
-# Definitions of special type checking related constructs.  Their definition
+# Definitions of special type checking related constructs.  Their definitions
 # are not used, so their value does not matter.
 
 overload = object()