iiif.io/guides/using_iiif_resources/ :: Do not link to test versions of Mirador

[mprove]

Page: https://iiif.io/guides/using_iiif_resources/
Now: A demo instance of Mirador can be found at https://projectmirador.org/.

This should not link to a defunctional test instance; esp. dropping manifest URLs does not work.

The link should point to https://projectmirador.org/embed/ instead.