update docstring for win_getpass to reflect code changes (GH-24967)

The code was updated in
0ec88b3
but the docstring was left untouched.

=> updated the docstring to reflect the code changes

Lib/getpass.py

@@ -95,7 +95,7 @@ def unix_getpass(prompt='Password: ', stream=None):
 
 
 def win_getpass(prompt='Password: ', stream=None):
-    """Prompt for password with echo off, using Windows getch()."""
+    """Prompt for password with echo off, using Windows getwch()."""
     if sys.stdin is not sys.__stdin__:
         return fallback_getpass(prompt, stream)