Fix missing iterations value in dnssec-guide

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.
73 jobs for !5317 with matthijs-iterations-typo-dnssec-guide in 20 minutes and 42 seconds (queued for 15 seconds)
latest detached
Status Job ID Name Coverage
  Autoconf
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  Precheck
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  Build
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passed clang:openbsd:amd64 #1916975
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passed clang:tsan #1916969
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passed gcc:alpine3.14:amd64 #1916954
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passed gcc:buster:amd64cross32 #1916959
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passed gcc:stretch:amd64 #1916957
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  Unit
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passed unit:gcc:softhsm2.4 #1916991
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passed unit:gcc:softhsm2.6 #1916992
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  System
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passed system:clang:buster:amd64 #1917010
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passed system:clang:freebsd12:amd64 #1917013
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passed system:clang:freebsd13:amd64 #1917015
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passed system:clang:tsan #1917009
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passed system:gcc:alpine3.14:amd64 #1916996
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passed system:gcc:softhsm2.4 #1917011
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passed system:gcc:softhsm2.6 #1917012
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passed system:gcc:stretch:amd64 #1916999
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  Docs
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  Postcheck
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