In the example https://graph.microsoft.com/v1.0/sites/site-id/drive/root:/{filename}:/content, the filename is the name of a new file. Quoting from the docs
This example uploads the string "The contents of the file goes here." to a file in the signed-in user's drive under FolderA named FileB.txt.
PUT /me/drive/root:/FolderA/FileB.txt:/content
Content-Type: text/plain
The contents of the file goes here.
In your case, the content will be the binary and filename will be something.docx.