Wrong index to make custom arrow be disabled

[vuquangpham]

Fix a bug: Wrong index

With old code, we check the current index of the slide based on input (just a number, similar to the index of change event).

But that's the wrong index (what happens when we change the groupCells of flkty to another, different to 1) and selectedIndex is just a number, but is-selected in DOM is more than 1. So I think that the selectedIndex that we used is just a current index of flickity dots (fix me if I'm wrong because I haven't read the flkty source code before).

So I have made a little change to make it work based on the is-selected position. Please check it! Many thanks.

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