Fix a bug in v3 API doc

This patch changes the wrong variable names 'addFixedIp' and 'networkId',
which were not used in V3 multinic API. We use 'add_fixed_ip' and
'network_id' in V3 multinic API.

Closes-bug: #1265716

Change-Id: I0c91764603d941df6d011a3b1266eb354c0bc54f
Merge "Fix a bug in v3 API doc"