update description of folders in upgrade.txt

upgrade.txt says:

1. Backup your database and Drupal directory - especially your "sites"
directory which contains your configuration file and added modules and
themes, any contributed or custom modules in your "modules" directory,
and your "files" directory which contains uploaded files. If other files
have modifications, such as .htaccess or robots.txt, those should be
backed up as well.

This isn't quite right any more.
"files" is now inside "sites", so it's sufficient to back that up.