Update README.md (backport #718)

[mergify]

This removes the outdated comment about not using FastDDS for topic monitoring between two machines.

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This is an automatic backport of pull request #718 done by Mergify.

[mergify]

Cherry-pick of ad559c0 has failed:

On branch mergify/bp/humble/pr-718
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	both modified:   topic_monitor/README.md

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