nbd: fix outdated qapi docs syntax for tls-creds

# QEMU instance could refer to them as "nbd:HOST:PORT:exportname=NAME".

#

# @addr: Address on which to listen.

# @tls-creds: (optional) ID of the TLS credentials object. Since 2.6

# @tls-creds: ID of the TLS credentials object (since 2.6).

# @tls-authz: ID of the QAuthZ authorization object used to validate

#             the client's x509 distinguished name. This object is

#             is only resolved at time of use, so can be deleted and