Update SlackUploadFileStep help-channel to use channel ID instead of …

[orcunbalcilar]

…channel name to fix misleading docs issue

Testing done

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• [ ] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [ ] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
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• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[orcunbalcilar]

Ok, I thought it would support both. But tried many times. Only id worked. Neither channel-name nor #channel-name was able to work. I had a look at the source code. Yes some support codes were there. But no result. Maybe this is a bug. Let me check it out again.