Add support for schema example in OAS3 parameters

[kmdsbng]

According to OAS3 Specification, an example element can be included in a schema object.

https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#schemaObject

I fixed to use example value under the schema object under the parameter object when it has set.

I'm using Stoplight Studio and I'm having the following problem, so I'd like you to import this pull request.

problem: When I set a example value of Query parameter in Stoplight Studio, it will output the value to parameters.schema.example in the OAS3 yaml file. If this file is read by dredd, an error will be displayed as "example" is not set.

I referenced #156 to make this pull request.

[kmdsbng]

coverall ci has failed. But I had no idea for fix this problem.
Can I do anything for fix this?

https://circleci.com/gh/apiaryio/api-elements.js/6742?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-checks-link

> nyc report --reporter=text-lcov | coveralls


/home/circleci/repo/node_modules/coveralls/bin/coveralls.js:19
      throw err;
      ^
Bad response: 422 {"message":"Couldn't find a repository matching this job.","error":true}
(Use `node --trace-uncaught ...` to show where the exception was thrown)