Fix broken example plugin documentation link

[rcgoodfellow]

The link to example plugin development docs 404s.

[miekg]

Or make it explugins? Can you redo this PR in the coredns/coredns repo? That's the canonical source of this text. Thanks for you contribution btw :)

[rcgoodfellow]

I could not find this particular markdown file in the coredns/coredns repo using a simple grep.

[coredns/coredns]$ grep -i "The canonical example plugin" -r .

If you give me a pointer to the correct location I'll resubmit the PR.

Thanks

[miekg]

oh no, this is manual. I'll just merge.