So it's great to see OOTB support for Teams when creating a Flow process. As you can see, all you have to do is to select from the available options (In this case Post message):
Now the key is to get the ID for the team you want to post to. Simply go to your team, select connectors and add a new Webhook connector.
The URL of web hook contains the GUID which is your Team ID:
https://outlook.office.com/webhook/[Team ID GUID]@...
Now that we have the id, simply past it and select the channel from drop down and complete your Flow:
To validate, add a new document and you should see a new post in that channel: