Remove usernames as Unicode use example

Amendment to https://github.com/edx/edx-documentation/pull/45
Turns out we do not support Unicode characters in usernames, though we still require UTF-8 encoded CSV files. This change removes "for example, usernames" in the "Creating a Unicode-encoded CSV File" topic so that we don't seem to imply that we support usernames with Unicode characters.
@explorerleslie @dan-f @lamagnifica please give thumbs if OK. I'd like to merge this today. Thanks!
FYI @WatsonEmily