Fix link in README.
Fix link in README.

mxm/spark-pr-dashboard,databricks/spark-pr-dashboard,JoshRosen/spark-pr-dashboard,databricks/spark-pr-dashboard,databricks/spark-pr-dashboard,d3borah/spark-pr-dashboard,d3borah/spark-pr-dashboard,mxm/spark-pr-dashboard,mxm/spark-pr-dashboard,JoshRosen/spark-pr-dashboard,d3borah/spark-pr-dashboard,mengxr/spark-pr-dashboard,mengxr/spark-pr-dashboard,mengxr/spark-pr-dashboard