Pull update for leaflet-tilelayer-wmts from original source

[petersmythe]

Source = https://github.com/alexandre-melard/leaflet.TileLayer.WMTS/blob/master/leaflet-tilelayer-wmts.js
Commit = alexandre-melard/leaflet.TileLayer.WMTS@52f3423

This is to fix the missing tilematrix prefix

[tomchadwin]

Thanks for the contribution. I didn't know WMTS was failing. Do you have an example?

[petersmythe]

Not any more, sorry. And sorry for the delay, I did not realise you were waiting for me.

The code in question qgis2web/js/leaflet-tilelayer-wmts.js was obviously taken from the original source = https://github.com/alexandre-melard/leaflet.TileLayer.WMTS/blob/master/leaflet-tilelayer-wmts.js and the problem that I was experiencing had been fixed by the commit referenced above Commit = alexandre-melard/leaflet.TileLayer.WMTS@52f3423

I thought that you might like to pull the same fix into your code.

You are welcome to simply reject and delete this PR if you wish.