index.xhtml: Give <html> attribute `lang="en"`

In `index.html`, give the root `<html>` tag the `lang="en"` attribute.

A run of <http://validator.w3.org/#validate_by_input> by @nyuszika7h
complained that `lang` is required if `xml:lang` is present on an
element “in no namespace”. Of course this element *is* in a namespace,
but I’m adding this anyway in the spirit of defensiveness.

I note that <http://validator.w3.org/#validate_by_uri> does not complain
about this or any other issue when run on <https://8573.github.io>.