Fix options for nova flavor-access-list

The text misses the --flavor option which is required:

$ nova help flavor-access-list
usage: nova flavor-access-list [--flavor <flavor>] [--tenant
<tenant_id>]

Add --flavor to nova flavor-access-list.

Change-Id: I374b2107b499be12f9e08e8d7deb0a07b4cacee1
Closes-Bug: #1315476
Merge "Fix options for nova flavor-access-list"