Fix atomic 64 b description (backport #17)

[mergify]

This is an automatic backport of pull request #17 done by Mergify.
Cherry-pick of 85efa4a has failed:

On branch mergify/bp/foxy/pr-17
Your branch is up to date with 'origin/foxy'.

You are currently cherry-picking commit 85efa4a.
  (fix conflicts and run "git cherry-pick --continue")
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
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	both modified:   CMakeLists.txt

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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