fixing yaml key typo for fleet.public-ip

According to https://coreos.com/docs/cluster-management/setup/cloudinit-cloud-config/ it should be "public-ip" not "public_ip"

Author: alex-sherwin

contrib/coreos/user-data

@@ -14,7 +14,7 @@ coreos:
   fleet:
     # We have to set the public_ip here so this works on Vagrant -- otherwise, Vagrant VMs
     # will all publish the same private IP. This is harmless for cloud providers.
-    public_ip: $private_ipv4
+    public-ip: $private_ipv4
     # allow etcd to slow down at times
     etcd_request_timeout: 3
   units: