Fix documentation: The correct key is 'api_url' (#771)
Fix documentation: The correct key is 'api_url' (#771)

See https://github.com/prometheus/alertmanager/blob/master/config/notifiers.go#L238
prometheus/docs,ddfreyne/prometheus-docs,prometheus/docs,chosenken/docs,ddfreyne/prometheus-docs,chosenken/docs,du2016/prometheus-docs,du2016/prometheus-docs,ddfreyne/prometheus-docs,prometheus/docs,du2016/prometheus-docs,chosenken/docs,chosenken/docs,ddfreyne/prometheus-docs