docs: fix incorrect parameter name in uploadV2 (channel -> channel_id)

[topkim993]

Summary

While implementing functionality as per the documentation, the channel parameter did not work as expected. Upon reviewing the official reference Upload a File - Slack API Documentation, I noticed that the correct parameter is channel_id. After making this change, the functionality worked as intended.

This PR proposes a fix to update the documentation to reflect the correct parameter, ensuring that other users do not encounter the same issue.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[seratch]

Thank you!