CloudEvent signature decorator should be snake case

[matthewrobertson]

"Cloud Event" is two words so the decorator should be snake case:

import functions_framework

@functions_framework.cloud_event
def hello_cloud_event(cloud_event):
  pass

[grant]

Note: I believe we should still refer to these events (i.e. in comments) as CloudEvent, but the appropriate snake case for this should be cloud_event as mentioned above.