Update -etcd-endpoints command line option in README

This patch updates the "key command line options" section in the README to reflect the changes regarding -etcd-endpoint(s) made in 74f9413deb00c0ca01bfb217aa56717b6edcfe64
Merge pull request #104 from patrickhoefler/feature/fix_etcd-endpoints_in_readme

Update -etcd-endpoints command line option in README