Update DNSSEC test link

[yubiuser]

What does this PR aim to accomplish?:

Updates the link to the DNSSEC test page. The former link is dead, but the author revived the page at new URL. See pi-hole/docs#871

How does this PR accomplish the above?:

Just changing the link.

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[jfb-pihole]

Why not list them both?

[yubiuser]

The former link is dead

Why should we keep it?

[jfb-pihole]

I withdraw my comment.