Fix callback-example.yaml

The runtime expression [must be quoted](http://www.yaml.org/spec/1.2/spec.html#id2788859) as it starts with a '{' - an [indicator character](http://www.yaml.org/spec/1.2/spec.html#c-indicator)

Also added 'openapi' and 'info' properties because they are required and, thus, the spec file is invalid without them.