data/settings: Update process_shutdown_filter example

Unit must be MB, since M is ambiguous and fails. And 10MB might be a bit too
little, so use 20MB.

Author: sirainen

data/settings.js

@@ -9829,7 +9829,7 @@ all of it to the OS). The syntax of the filter is described in
 For example:
 
 \`\`\`
-process_shutdown_filter = "event=mail_user_session_finished AND rss > 10M"
+process_shutdown_filter = "event=mail_user_session_finished AND rss > 20MB"
 \`\`\``
 	},