Remove help-desk email address from aboutTor.dtd

aboutTor.dtd1 still uses help@rt.torproject.org, this should be removed until the help-desk is reopened.

<!ENTITY aboutTor.failure3prefix.label "For assistance, please contact "> <!ENTITY aboutTor.failure3Link "help@rt.torproject.org"> <!ENTITY aboutTor.failure3suffix.label ".">

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