Correct "To create a server" SDK example

"To create a server" SDK example incorrectly used net_id instead of
net.id when assigning the network identifier to a NIC.

Change-Id: Ica993938c57e048f56c03c777ccdebae7ffdedfa
Closes-Bug: #1390347
backport: juno
Merge "Correct "To create a server" SDK example"