This way works:

alertmanager:
  config:
    global:
      resolve_timeout: 5m
    route:
      receiver: 'slack-notifications'
      group_by: ['alertname']
      group_wait: 10s
      group_interval: 5m
      repeat_interval: 1h
    receivers:
      - name: 'slack-notifications'
        slack_configs:
          - api_url: 'https://hooks.slack.com/services/XXXXXXXX/YYYYYYYYY/ZZZZZZZZZZZZZZZZZZ'
            channel: '#alert-channel'
            send_resolved: true
            username: 'Prometheus'
            text: "{{ .CommonLabels.alertname }}: {{ .CommonAnnotations.description }}"
...