api-ref: fix server_id in metadata docs
api-ref: fix server_id in metadata docs

The api-ref was saying that the server_id was in the body of the
server metadata requests but it's actually in the path for all
of the requests.

Change-Id: Icdecd980767f89ee5fcc5bdd4802b2c263268a26
Closes-Bug: #1641331

mahak/nova,jianghuaw/nova,phenoxim/nova,mikalstill/nova,rahulunair/nova,phenoxim/nova,klmitch/nova,gooddata/openstack-nova,jianghuaw/nova,hanlind/nova,mahak/nova,mikalstill/nova,openstack/nova,rahulunair/nova,klmitch/nova,hanlind/nova,gooddata/openstack-nova,vmturbo/nova,vmturbo/nova,gooddata/openstack-nova,Juniper/nova,Juniper/nova,jianghuaw/nova,Juniper/nova,mahak/nova,klmitch/nova,hanlind/nova,klmitch/nova,rajalokan/nova,rajalokan/nova,gooddata/openstack-nova,rajalokan/nova,rajalokan/nova,mikalstill/nova,openstack/nova,Juniper/nova,rahulunair/nova,vmturbo/nova,openstack/nova,vmturbo/nova,jianghuaw/nova