Provide better description of package upload for corrections

[fosterfarrell9]

A new user complained that the explanation on how the bulk upload for tutors works (given in a "helpdesk") may lead to confusion, in particular since the upload button there is labeled "File"/"Datei" which might suggest that you have to pack all the corrections in one file, like a zip. So: Improve the text in the helpdesk and change the name of the button to "Files".