Cursor.listen not working with url strings

[pabaucom]

Found when using the example code in cursor.md, it does not work as expected. The callback is never called.

From the current cursor.md:

-- not work
cursor.listen("#cursor", cursor.DRAG_START, function(message_id, message)
    -- prevent dragging of blue aliens
    if message.group == hash("blue") then
        return true -- return true to block the event
    end
end)

It does work as expected if we use the full url instead of the url string "#cursor":

-- works
cursor.listen(msg.url("#cursor"), cursor.DRAG_START, function(message_id, message)
    -- prevent dragging of blue aliens
    if message.group == hash("blue") then
        return true -- return true to block the event
    end
end)

The URL is also how it is in the example controller.script

I was going to submit a PR for cursor.md with updated example code, but then I saw that it says cursor.listen() should take either a url or string, so I'm not sure if the doc needs fixed or cursor.lua needs fixed.