Update js/examples/maplibre.html

[paulovieira]

Some small changes to make the this important example more clear:

• wrap all the code in an async function, so that we can use await p.getHeader()
• some small changes for coherence (double quotes, etc)

If there is interest I can make these same changes in the other 2 examples (maplibre_raster_dem.html and leaflet.html)

[bdon]

I don't view using await as inherently more correct than using .then() in this context of a simple script using the IIFE script-includes. Is there another reason why this would be better?

[bdon]

Fixed formatting in #438 , thanks.