Fix-up README to use METAL_AUTH_TOKEN consistent with what the packag…

…e would expect from an environment variable-- this isn't breaking example, but as a debug step is clearer/more consistent.

Signed-off-by: Joseph D. Marhee <jmarhee@interiorae.com>

README.md

@@ -88,7 +88,7 @@ To monitor the Docker debugging details and the Equinix Metal API calls:
 go build
 PACKNGO_DEBUG=1 PATH=`pwd`:$PATH docker-machine \
   --debug create -d metal \
-  --metal-api-key=$METAL_TOKEN \
+  --metal-api-key=$METAL_AUTH_TOKEN \
   --metal-project-id=$METAL_PROJECT \
   foo
 ```