Please update the instructions with how to revert back to the cloud

[karmakazi]

The :spark: cloud public key file is here: https://s3.amazonaws.com/spark-website/cloud_public.der

Place your core in DFU-mode (flashing yellow)

download the public key file here: https://s3.amazonaws.com/spark-website/cloud_public.der

on the command-line enter the following:

spark keys server cloud_public.der

Take out the apiUrl line in spark.config.json