doc: fix xfr example. (#1062)
doc: fix xfr example. (#1062)

* doc: fix xfr example.

The currently example code has a data race, put in the proper code.

Fixes: #1061

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>

* Feedback

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>

miekg/dns