Update sprintf arguments

Updated the sprintf arguments for posting files to match the order used in Pushbullet API documentation and for consistency with the order of arguments used for other types: note, link, address.

Of importance, it appears as though an %s was missing, resulting in the shift in arguments. This appears to have resolved #5 in the tests run:
```r
str(fromJSON(pbPost(type="file", url=system.file("DESCRIPTION", package="RPushbullet"),recipients = 1)[[1]]))
...
 $ file_name                : chr "DESCRIPTION"
 $ file_type                : chr "text/plain"
 $ file_url                 : chr "https://s3.amazonaws.com/pushbullet-uploads/ujAcBbCHoAK-szNPoS6OH2lvOhybeqFzBjD6joH6d8W0//Library/Frameworks/R.framework/Versio"| __truncated__
```