writing-specs.md: Add examples for async/promise specs

That & I fixed the jasmine docs link since you redirected to the __2.0__ docs. When I was reading your guide for the first time I was sad and angry at myself that I couldn't get my `done()` tests to work. :wink:
```coffee
atom.workspace.open('c.coffee').then (editor) -> 
  expect(editor.getPath()).toContain 'c.coffee'
````

I think that would better match style used in Atom's promise callbacks.