chore: fix the incorrect Slack link in our readme #3990

Job	Run time
lint	4m 6s
Running with Python 3.7	8m 44s
Python Build (Python 3.10 PyArrow latest)	5m 49s
Running with Python 3.8	6m 39s
Python Benchmark	11m 17s
Running with Python 3.9	6m 12s
Python Build (Python 3.7 PyArrow 8.0.0)	10m 5s
PySpark Integration Tests	4m 51s
Running with Python 3.10	6m 25s
Running with Python 3.11	6m 21s
	1h 10m 29s

Provide feedback