docs bug: successful submission missing "begins"

https://trolie.energy/example-narratives/submitting-forecasts.html#pushing-inputjson-to-trolie-with-curl

also update the spec to have a better description here, if possible https://github.com/trolie/spec/blob/1.0.0-wip/docs/_data/components/schemas/array-max-monitored-elements.yaml#L224