Clarified that callbacks are FineUploader option.

[Martinsos]

Thanks for this awesome library!
I have been using it on a project to perform upload to Azure from browser with completely custom UI.

Brief description of the changes

I had hard time figuring out where to define callbacks while reading docs, as it is never explicitly stated that callbacks is a FineUploader option. I ended up trying different things and at the end it worked if I would provide as FineUploader option. Therefore, I thought it would be useful to explicitly state that in docs, and that is what this PR does.

What browsers and operating systems have you tested these changes on?

None, because I did not manage to run building of documentation. I inspected Makefile and tried downloading docfu manually but realized that should probably not be the way to build it and stopped there, because I don't have a good idea how to build it.
Therefore, I am not sure how the PR is going to render. If you could point me toward the way to do this I will, or you can try it out and fix any errors.

Have you written unit tests? If not, explain why.

No, because I changed only documentation.

[rnicholus]

Looks good to me. Thanks for this!

[Martinsos]

Awesome, thanks!
Question for the future, if I change a piece of docs, is there any easy way to render it and check what it looks like?