Fixed a small typo in the private API's scan_file method.

The POST request in scan_file (PrivateApi class) uses file=files. From the
private API documentation it should be files=files (files with an 's'; see
https://www.virustotal.com/en/documentation/private-api/#scan)
Merge pull request #2 from adrianherrera/private-scan-file-fix

Fixed a small typo in the private API's scan_file method