Fix element.query_selector

[agj]

Hi! plinth/browser/element's query_selector wasn't working for me, and I realized that the code was still using the outdated JavaScript API.

Here's the error message I got on the browser's console:

Uncaught ReferenceError: Ok is not defined
    querySelector element_ffi.mjs:233
    on_clicked_document app.gleam:1

So I changed new Ok to Result$Ok (like in #37) and now it works for me.

By the way, by grepping I didn't find any other case similar to this.

[CrowdHailer]

Thank you. looks good to me. I wonder how this one passed by the previous update

[agj]

Maybe improper use of grep? 😄