Update README.md for finalizeTwoFactor

[matvejs16]

String type is correct instead of Buffer

Not working example:

// secret is string
client.finalizeTwoFactor(Buffer.from(secret), emailCode, (err) => {})

Correct example:

// secret is string
client.finalizeTwoFactor(secret, emailCode, (err) => {})

[DoctorMcKay]

finalizeTwoFactor does in fact accept a Buffer in addition to a string. Buffer.from is failing because you didn't specify an encoding, which defaults to 'utf8'. You likely want to specify 'base64' as your encoding.