fix typo in override() docstring (#305)

backport of python/cpython@12c7e9d

Author: AlexWaygood

src/typing_extensions.py

@@ -2252,7 +2252,7 @@ def override(arg: _F, /) -> _F:
         Usage:
 
             class Base:
-                def method(self) -> None: ...
+                def method(self) -> None:
                     pass
 
             class Child(Base):