When uploading files, use "parent_id" instead of "folder_id"

When you upload a file, the documentation states that you should specify a field called `parent_id` which is the folder id of the folder into which the file should be uploaded.

The API also accepts `folder_id`; but `parent_id` is the preferred name for the field. So, this commit changes the Java SDK to use `parent_id` instead of `folder_id`.