Correct Kovri user guide link from moneropedia articles

Merged el00ruobuob requested to merge el00ruobuob/monero-site:souscription-link-user-guide into master

I found this while translating "subscription" article.
Link is refering to an old tree and is therfore "file not found" on the other end.

Should we link to getkovri, or github is still the target @anonimal?

See !841 (closed)

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