Replacing "upload" with "download"

&@wibeasley

I am not sure if this is the best way to recommend changes to the DocumentationPeek pdf, but I did notice a couple of typos that might confuse the inexperienced programmer.

Under the heading for the redcap_download_file_oneshot function, the description reads:
````
Description:
    This function uses REDCap's API to upload a file
````

When I think you meant "This function uses REDCap's API to *download* a file.
Merge pull request #45 from ARPeters/patch-1

Replacing "upload" with "download"