fix etcd sha256 in README.md

following the instructions I get a different hash:

$ wget -q https://github.com/coreos/etcd/releases/download/v0.5.0-alpha.4/etcd-v0.5.0-alpha.4-linux-amd64.aci
$ gzip -dc etcd-v0.5.0-alpha.4-linux-amd64.aci > etcd-v0.5.0-alpha.4-linux-amd64.tar
$ sha256sum etcd-v0.5.0-alpha.4-linux-amd64.tar
f9215c18b86f406c7cec4c7b45fd8752b5bfd1a492507d647821c2ce593fbf31 etcd-v0.5.0-alpha.4-linux-amd64.tar
Merge pull request #229 from filippog/fix/etcd-sha256

fix etcd sha256 in README.md