Update issue tracker URL in docs

Author: srowen

docs/_config.yml

@@ -8,5 +8,5 @@ SPARK_VERSION_SHORT: 1.0.0
 SCALA_BINARY_VERSION: "2.10"
 SCALA_VERSION: "2.10.4"
 MESOS_VERSION: 0.13.0
-SPARK_ISSUE_TRACKER_URL: https://spark-project.atlassian.net
+SPARK_ISSUE_TRACKER_URL: https://issues.apache.org/jira/browse/SPARK
 SPARK_GITHUB_URL: https://github.com/apache/spark