samples/zoap_client: Fix using the wrong timeout for retransmissions

It was assumed that the unit was microseconds, it is in miliseconds,
the same unit that is stored in the timeout field of the pending
transmission.

Change-Id: Ia99f363c7de4ec76a7ed229cb94a9964bcf609aa
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>