[GOBBLIN-1539] Fix community Slack link in README
[GOBBLIN-1539] Fix community Slack link in README

Closes #3391 from autumnust/fix_slack_link

shirshanka/gobblin,linkedin/gobblin,arjun4084346/gobblin,shirshanka/gobblin,shirshanka/gobblin,shirshanka/gobblin,linkedin/gobblin,linkedin/gobblin,jack-moseley/gobblin,jack-moseley/gobblin,jack-moseley/gobblin,jack-moseley/gobblin,arjun4084346/gobblin,arjun4084346/gobblin,shirshanka/gobblin,arjun4084346/gobblin,linkedin/gobblin,linkedin/gobblin,arjun4084346/gobblin,linkedin/gobblin,jack-moseley/gobblin