Update the Slack site [for newcomers]

[luocooong]

trafficstars

Describe the bug

As the short-link (https://bit.ly/2VM0XS8) has expired, we cannot click on the "join" character to access the Slack channel.

I recommended that you can remove the site of "join" directly, the "Slack Channel" is also provide the correct site.

https://github.com/apache/drill/blob/239a8f8336936ac51707fbf4a73da86a1156cd59/README.md?plain=1#L29

Expected behavior

Update the README.md, let the "join" character not use the hyperlinks.

Screenshots

Additional context

1. If you would like to know about the process of contribution, refer to Help beginners to contribute to open source projects.
2. Please let us know if you need any help, mailing list and Slack.