Fixes typo in idlelib/idle_test/example_stub.pyi (python#122520)

---------
Co-authored-by: Terry Jan Reedy <[email protected]>

Author: 2 people

Lib/idlelib/idle_test/example_stub.pyi

@@ -1,4 +1,4 @@
-" Example to test recognition of .pyi file as Python source code.
+# An example file to test recognition of a .pyi file as Python source code.
 
 class Example:
     def method(self, argument1: str, argument2: list[int]) -> None: ...