os/booting-on-gce: Fix environment variable name
os/booting-on-gce: Fix environment variable name

Fix the Google Compute Engine ignition config to use the correct metadata
environment variable. COREOS_GCE_IPV4_LOCAL_0 should be
COREOS_GCE_IP_LOCAL_0.

atxwebs/docs,coreos/docs,endocode/coreos-docs,joshix/docs