Fixes link to issue tracker in documentation page "Contributing to Spark".

docs/_config.yml

@@ -7,3 +7,4 @@ SPARK_VERSION: 0.7.0-SNAPSHOT
 SPARK_VERSION_SHORT: 0.7.0
 SCALA_VERSION: 2.9.2
 MESOS_VERSION: 0.9.0-incubating
+SPARK_ISSUE_TRACKER_URL: https://spark-project.atlassian.net

docs/contributing-to-spark.md

@@ -15,7 +15,7 @@ The Spark team welcomes contributions in the form of GitHub pull requests. Here
   But first, make sure that you have [configured a spark-env.sh](configuration.html) with at least
   `SCALA_HOME`, as some of the tests try to spawn subprocesses using this.
 - Add new unit tests for your code. We use [ScalaTest](http://www.scalatest.org/) for testing. Just add a new Suite in `core/src/test`, or methods to an existing Suite.
-- If you'd like to report a bug but don't have time to fix it, you can still post it to our [issue tracker](https://spark-project.atlassian.net), or email the [mailing list](http://www.spark-project.org/mailing-lists.html).
+- If you'd like to report a bug but don't have time to fix it, you can still post it to our [issue tracker]({{site.SPARK_ISSUE_TRACKER_URL}}), or email the [mailing list](http://www.spark-project.org/mailing-lists.html).
 
 # Licensing of Contributions