References to Swagger should say OpenAPI

[dbenhur]

Starting January 1st 2016 the Swagger Specification has been donated to the Open API Initiative (OAI) and is the foundation of the OpenAPI Specification.

All references to Swagger in our docs should reflect its current name (or both, eg "OpenAPI (fka Swagger)") and link the OpenAPI-Specification.

[sanathkr]

We want to keep the nomenclature in this spec close to the language in CloudFormation's documentation to avoid confusion. We can make the change as soon as CloudFormation doc changes

[shefulloflight]

The CFN docs are now updated.