Correct typo in request specification

[The `grunt-saucelabs` npm module](https://github.com/axemclion/grunt-saucelabs) demonstrates [the correct parameter name](https://github.com/axemclion/grunt-saucelabs/blob/990f3cf93b132333501533dc20e28b27aa772ef6/src/Job.js#L85).

Commit message:

> The parameter `tunnel_identifier` is ignored by Sauce Labs today. Instead, it honors the value specified as `tunnel-identifier`.