Fix description for 'Freezer sessions management' and 'Session-Job association' in README.rst
Fix description for 'Freezer sessions management' and 'Session-Job
association' in README.rst

Incorrect description for 'Freezer sessions management' and
'Session-Job association' in README.rst, change the description
of 'DELETE' from 'adds the job to the session' to 'removes
the job from the session'.

Change-Id: I1d2be9f40982735e8985bbfb2bbc079620d15873

szaher/freezer-api,szaher/freezer-api,szaher/freezer-api,openstack/freezer-api,openstack/freezer-api,szaher/freezer-api,openstack/freezer-api,openstack/freezer-api