ODF -> ODT

[jackycute]

From: hackmdio/codimd#519

@zuphilip
The label for the LibreOffice document is currently ODF but it should be rather ODT in the online editor.

[brlin-tw]

What about ODT (ODF)?
Excluding the ODF naming will impede the current promotion of OpenDocumentFormat in Taiwan as it'll cause confusion to the users due to the unfamiliar filename extension.