chore(acm): update docs (#2028)

Update class name in docs, remove outdated paragraph.

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

@@ -5,7 +5,7 @@ can be used in CloudFront and ELB.
 
 ### DNS-validated certificates
 
-The `DNSValidatedCertificateRequest` class provides a Custom Resource by which
+The `DnsValidatedCertificateRequest` class provides a Custom Resource by which
 you can request a TLS certificate from AWS Certificate Manager that is
 automatically validated using a cryptographically secure DNS record. For this to
 work, there must be a Route 53 public zone that is responsible for serving
@@ -44,9 +44,6 @@ in the email. The email addresses use will be:
 * [email protected]
 * [email protected]
 
-DNS validation is possible in ACM, but is not currently available in CloudFormation.
-A Custom Resource will be developed for this, but is not currently available.
-
 Because of these blocks, it's probably better to provision your certificates either in a separate
 stack from your main service, or provision them manually. In both cases, you'll import the
 certificate into your stack afterwards.