Update doc: Add missing attribute in RequestModel

[kavindasr]

Issue #, if available:
In doc ReuqestModel has only two attributes. But RequestModel has an attribute call ValidateBody
Description of changes:

Description of how you validated changes:

Checklist:

• Add/update unit tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• Add/update integration tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

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

[jfuss]

@kavindasr Thank you for the PR however, we are going to remove this file from the repo. Reason is stated here: #2334 but we have docs we maintain with each release and deployed to the AWS Doc site. Instead of keeping the repo and AWS Docs updated, we will keep the AWS Docs updated.

Again thank you for taking the time to submit this.