[devscripts/prepare_manpage] Fix deprecated escape sequence on py36

devscripts/prepare_manpage.py

@@ -8,7 +8,7 @@ import re
 ROOT_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 README_FILE = os.path.join(ROOT_DIR, 'README.md')
 
-PREFIX = '''%YOUTUBE-DL(1)
+PREFIX = r'''%YOUTUBE-DL(1)
 
 # NAME