Fix --output-file option

When opening the file for binary writing, the file mode was incorrectly
specified.

The file can now be successfully written with:
```shell
python create_pdfs.py \
    --output-file report.csv
    --course-id 'edX/DemoX/Demo_Course' \
    --no-upload \
    --name 'Guido'
```
That seems fine and I watched you test it. Why not update the help output while you're at it?  Even two more words to clarify would help avoid confusion.