nbd: fix outdated qapi docs syntax for tls-creds

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190227162035.18543-4-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

qapi/block.json

@@ -224,7 +224,7 @@
 # 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