Add "update fallbacks" to "Preliminaries" in Tor's doc/HACKING/ReleasingTor.md

When we added fallback directory mirrors, we should have also listed them as something we need to update every major release:

  1. Early in the alpha series for each new major release, at least a month before the code freeze, update the list of fallback directory mirrors using the instructions in: https://trac.torproject.org/projects/tor/wiki/doc/UpdatingFallbackDirectoryMirrors
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