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

[orcunbalcilar]

…channel name to fix misleading docs issue

Testing done

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[timja]

This isn't correct, whilst channel ids will be more efficient channel names are supported just fine:
https://github.com/jenkinsci/slack-plugin/blob/master/src/main/java/jenkins/plugins/slack/cache/SlackChannelIdCache.java#L69

[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.