bpo-38351: Modernize email examples from %-formatting to f-strings (GH-17162)

diff --git a/Doc/includes/email-simple.py b/Doc/includes/email-simple.py
index f69ef40..07dc30f 100644
--- a/Doc/includes/email-simple.py
+++ b/Doc/includes/email-simple.py
@@ -12,7 +12,7 @@
 
 # me == the sender's email address
 # you == the recipient's email address
-msg['Subject'] = 'The contents of %s' % textfile
+msg['Subject'] = f'The contents of {textfile}'
 msg['From'] = me
 msg['To'] = you