Fix missing iterations value in dnssec-guide (9.16)

In the "Migrating from NSEC to NSEC3" section, it says:

    dnssec-policy "standard" {
        nsec3param iterations optout no salt-length 16;
    };

There should be an integer after "iterations". Based on the following text, the number of iterations should be 10.

(cherry picked from commit 9e109191)

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