docs(route53): fix broken heading (#31693)

### Issue # (if applicable)

None.

### Reason for this change

The headings are set incorrectly, causing the table of contents to be misaligned.

<img width="244" alt="スクリーンショット 2024-10-08 20 13 13" src="https://github.com/user-attachments/assets/b08ab461-031b-4ffb-9937-3ab4ad0cddef">

### Description of changes

Remove heading and made some minor adjustments to the document.

### Description of how you validated changes

None

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: badmintoncryer

packages/aws-cdk-lib/aws-route53/README.md

@@ -105,11 +105,14 @@ new route53.ARecord(this, 'ARecord', {
 ```
 
 To create an A record of type alias with target set to another record created outside CDK:
-### This function registers the given input i.e. DNS Name(string) of an existing record as an AliasTarget to the new ARecord. To register a target that is created as part of CDK use this instead https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_route53_targets-readme.html
 
-```ts
+This function registers the given input i.e. DNS Name(string) of an existing record as an AliasTarget to the new ARecord. To register a target that is created as part of CDK use this instead.
+
+Detailed information can be found in the [documentation](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_route53_targets-readme.html).
 
+```ts
 declare const myZone: route53.HostedZone;
+
 const targetRecord = 'existing.record.cdk.local';
 const record = route53.ARecord.fromARecordAttributes(this, 'A', {
   zone: myZone,