compute: fix typo in update_security_groups()
compute: fix typo in update_security_groups()

RESP BODY: {"badRequest": {"message": "Missing parameter security_group", "code": 400}}

According to Compute API[0], key should be "security_group" (with
underscore).

Just fixing this small typo that was there from day 0.

[0] https://developer.openstack.org/api-ref/compute/#update-security-group

Change-Id: I0f8f4007dac6cbea94068f1654ea2f3d550c5583
Signed-off-by: Igor Gnatenko <c0919744cf82c4c971a09c96755a0dea3683b87c@gooddata.com>

stackforge/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,dtroyer/python-openstacksdk,openstack/python-openstacksdk,openstack/python-openstacksdk