Update pmtiles.md; add glyphs download location

[jamesrr39]

Hello and thank you for your blog post on setting up PM Tiles with MapLibre, I was able to follow it to get my own setup running!

I was also able to find the location for downloading the font glyphs, so that the page can be fully self hosted without relying at all on any third party/CDN. This PR adds that link.