[cloudwatch] Composite Alarm ARN is incorrect

[Khufu-I]

Cloudwatch Composite Alarm ARN generated by CompositeAlarm construct is incorrect.

Reproduction Steps

import cdk from 'monocdk';
import cloudwatch from 'monocdk/aws-cloudwatch.js';

const availabilitySharpDropAlarm = new cloudwatch.Alarm(this, 'AvailabilitySharpDropAlarm', {
  evaluationPeriods: 5,
  metric: availabilityMetric.with({ period: cdk.Duration.minutes(1) }),
  threshold: 50,
  alarmName: `Synthetics-Alarm-AvailabilitySharpDrop`,
});
const availabiltyAlarm = new cloudwatch.CompositeAlarm(this, 'AvailabilityAlarm', {
  compositeAlarmName: `availabilty`,
  alarmDescription: `Composite alarm`,
  alarmRule: cloudwatch.AlarmRule.anyOf(availabilitySharpDropAlarm),
});
//availabilityAlarm.alarmArn is incorrect

What did you expect to happen?

The separator between resource name and composite alarm name should be :

       "Fn::Join": [
            "",
            [
              "arn:",
              {
                "Ref": "AWS::Partition"
              },
              ":cloudwatch:us-west-2:945593679341:alarm:availabilty"
            ]
          ]

What actually happened?

The separator between resource name and composite alarm is /

       "Fn::Join": [
            "",
            [
              "arn:",
              {
                "Ref": "AWS::Partition"
              },
              ":cloudwatch:us-west-2:945593679341:alarm/availabilty"
            ]
          ]

Environment

• CLI Version : aws-cli/2.0.46 Python/3.7.4 Darwin/19.6.0 exe/x86_64
• Framework Version: monocdk 1.69.0
• Node.js Version: 12.9.1
• OS : macOS 10.15.7
• Language (Version): JavaScript 2020

Other

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This is 🐛 Bug Report

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