docs(aws-apigatewayv2-alpha): update domain name documentation (#23462)

----
I am requesting an update to `apigwv2.DomainName`, which accepts `domainName` as a prop. Current documentation has an error, it currently says:
```
const domainName = 'example.com';

const dn = new apigwv2.DomainName(this, 'DN', {
  domainName,
  certificate: acm.Certificate.fromCertificateArn(this, 'cert', certArn),
});
```
When you run the above ☝️ it errors out. 
It should read:
```
const dn = new apigwv2.DomainName(this, 'DN', {
  domainName: domainName,
  certificate: acm.Certificate.fromCertificateArn(this, 'cert', certArn),
});
```

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Author: jedrekdomanski

packages/@aws-cdk/aws-apigatewayv2/README.md

@@ -184,7 +184,7 @@ const certArn = 'arn:aws:acm:us-east-1:111111111111:certificate';
 const domainName = 'example.com';
 
 const dn = new apigwv2.DomainName(this, 'DN', {
-  domainName,
+  domainName: domainName,
   certificate: acm.Certificate.fromCertificateArn(this, 'cert', certArn),
 });