fix documentation

[elasticdotventures]

on the AWS managed docs page the receiver config has two issues with the sample configuration for receivers is wrong.

https://docs.aws.amazon.com/prometheus/latest/userguide/AMP-alertmanager-receiver-config.html

firstly, the format is not key: key, value: value

I've verified the prometheus docs are correct:

• https://prometheus.io/docs/alerting/latest/configuration/#sns_configs attributes are: key1: value1 key2: value2

secondly, the proposed indentation for the sigv4: should not have a - .. I think that would actually break it. This is taken from a working config:

  receivers:
    - name: 'opsgenie-sns'
      sns_configs:
        - topic_arn: "replace_arn"
          send_resolved: false
          subject: '{{ template "opsgenie.default.subject" .}}'
          message: ''
          sigv4:
            region: "${region}"
          attributes:
            tags: '{{ template "__opsgenie_tags" .Alerts.Firing }}'
            teams: '{{ template "__opsgenie_teams" .Alerts.Firing }}'
            priority: '{{ template "__opsgenie_priority" .CommonLabels.SortedPairs }}'