Makes example_app.models.Product.photo.upload_to a string instead of bytes

[vbawa]

Hello,

When playing around with some changes to django-silk, I found an error in the example app.

Steps to repro:

1. Run Django server
2. Go to http://localhost:8000/admin/example_app/blind/add/ and upload a photo, and choose name.
3. Click Save

Upon saving, the following (truncated) server exception occurred:

....
File "/home/veer/Projects/silk/venv/lib/python3.5/site-packages/django/db/models/fields/files.py", line 86, in save
  name = self.field.generate_filename(self.instance, name)
File "/home/veer/Projects/silk/venv/lib/python3.5/site-packages/django/db/models/fields/files.py", line 304, in generate_filename
  dirname = datetime.datetime.now().strftime(self.upload_to)
TypeError: strftime() argument 1 must be str, not bytes

It seems like this is down to a simple typo in the 'upload_to' params of example_app.models.Product.photo, passing in bytes instead of a string. I've fixed it in this teeny PR.

I read the Contributing guidelines and the README, but please let me know if I missed something and there's some other way I should be approaching this (like filing an associated issue before sending a PR).

Thanks!

[vbawa]

I just thought to check whether this bug happens on Python2.7 as well as Python3.5. It doesn't appear to cause the same crash in Python2.7. I did verify that my modification runs correctly on both Python2.7 and Python3.5, though. I think this comes down to the fact that in Python3, strftime stopped being lenient about translating a bytes input into a string before processing.