fix typo in property getter selector

[mathieudutour]

Quick repro in sketch:

let sketch = require('sketch')
let document = sketch.getSelectedDocument()

console.log(document.sketchObject.documentEdited)

used to throws Unable to parse method encoding for method documentEdited: of class MSDocument.

It will now print a boolean.

Note that properties aren't methods and as such, don't need to be called. It shouldn't break anything because methods are still matched first.

[ccgus]

Thanks for the patch!