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.

Author: tfesenko

examples/v3.0/callback-example.yaml

@@ -1,3 +1,7 @@
+openapi: 3.0.0
+info: 
+  title: Callback Example
+  version: 1.0.0
 paths:
   /streams:
     post:
@@ -32,7 +36,7 @@ paths:
         onData:
           # when data is sent, it will be sent to the `callbackUrl` provided
           # when making the subscription PLUS the suffix `/data`
-          {$request.query.callbackUrl}/data:
+          '{$request.query.callbackUrl}/data':
             post:
               requestBody:
                 description: subscription payload