RedfishPkg: update Readme.md

RedfishClientPkg is moved from edk2-staging repository to
edk2-redfish-client repository. Update the link in Readme.md
to new location.

Signed-off-by: Nickle Wang <nicklew@nvidia.com>
Cc: Abner Chang <abner.chang@amd.com>
Cc: Igor Kulchytskyy <igork@ami.com>
Reviewed-by: Abner Chang <abner.chang@amd.com>