Flatten example values in schema generation

[alampros]

When using joi.any.example(), the generated schema contains a map with a value prop. This PR flattens the example properties to only the value of the value property.

Given this joi schema:

const responseSchema = Joi.object().keys({
  responseObjectKey: Joi.string().example('some example string'),
})

Diff of Generated Model Definition With This Change:

...
"definitions": {
  "SomeModel": {
       "properties": {
         "responseObjectKey": {
           "type": "string",
-          "example": {
-            "value": "some example string"
-          }
+          "example": "some example string"
         }
       }
     }
  }
}
...

PS - Thank you for the amazing work on this project! 🤩

[coveralls]

Coverage decreased (-0.08%) to 99.357% when pulling c9643ed on alampros:fix/flatten-example-values into f725498 on z0mt3c:master.