Update buster install manual naming scheme for the HTML files

Keep the install manual naming scheme for the PDF and plain text files.

The HTML install files were renamed from index.html.en to index.en.html
but the PDF and plain text files were not renamed.

Reported-by: Michael Kesper <mkesper@schokokeks.org>
Reported-in: <d571917a-93a6-1df3-fe09-bd0bedd47974@schokokeks.org>