Update README.md

former link shows:
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[root@localhost mongo-cxx-driver-v2.2]# git clone git@github.com:telefonicaid/fiware-orion.git
Cloning into 'fiware-orion'...
Warning: Permanently added the RSA host key for IP address '192.30.252.131' to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
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so I changed the link in order to allow downloading the repo