Fix the link to the rally docs in README.rst
Fix the link to the rally docs in README.rst

The links to the rally docs are invalid.
This patch fixes it.

Change-Id: I8713a8cdf317d385e770528111b0ac376f89391f
Closes-Bug: #1718819

huntxu/neutron,noironetworks/neutron,eayunstack/neutron,openstack/neutron,openstack/neutron,mahak/neutron,noironetworks/neutron,eayunstack/neutron,huntxu/neutron,mahak/neutron,openstack/neutron,mahak/neutron