[3.14] Docs: fix docstring of email.message.Message.add_header (GH-134355) (#135339)

Docs: fix docstring of `email.message.Message.add_header` (GH-134355)
(cherry picked from commit c23eec2)

Co-authored-by: Alexander Shadchin <shadchin@yandex-team.com>

Author: miss-islington

Lib/email/message.py

@@ -564,7 +564,7 @@ def add_header(self, _name, _value, **_params):
 
         msg.add_header('content-disposition', 'attachment', filename='bud.gif')
         msg.add_header('content-disposition', 'attachment',
-                       filename=('utf-8', '', Fußballer.ppt'))
+                       filename=('utf-8', '', 'Fußballer.ppt'))
         msg.add_header('content-disposition', 'attachment',
                        filename='Fußballer.ppt'))
         """