Update readme.md

[sandervandevelde]

Both an IoT hub and each device in it have connectionstrings.

I renamed the to to prevent confusion.

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• [x] I have read the contribution guidelines.
• [ ] I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• If this is a modification that impacts the behavior of a public API
  • [ ] I edited the corresponding document in the devdoc folder and added or modified requirements.

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[danhellem]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 2 pipeline(s).

[danhellem]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 2 pipeline(s).