[SPARK-17567][Docs] Use valid url to Spark RDD paper

[keypointt]

https://issues.apache.org/jira/browse/SPARK-17567

What changes were proposed in this pull request?

Documentation (http://spark.apache.org/docs/latest/api/scala/#org.apache.spark.rdd.RDD) contains broken link to Spark paper (http://www.cs.berkeley.edu/~matei/papers/2012/nsdi_spark.pdf).

I found it elsewhere (https://www.usenix.org/system/files/conference/nsdi12/nsdi12-final138.pdf) and I hope it is the same one. It should be uploaded to and linked from some Apache controlled storage, so it won't break again.

How was this patch tested?

Tested manually on local laptop.

[keypointt]

Just to be consistent with the url at http://spark.apache.org/research.html

Resilient Distributed Datasets: A Fault-Tolerant Abstraction for In-Memory Cluster Computing. Matei Zaharia, Mosharaf Chowdhury, Tathagata Das, Ankur Dave, Justin Ma, Murphy McCauley, Michael J. Franklin, Scott Shenker, Ion Stoica. NSDI 2012. April 2012. Best Paper Award.

change it to be http://people.csail.mit.edu/matei/papers/2012/nsdi_spark.pdf

[keypointt]

I just noticed, that at http://spark.apache.org/research.html

Spark offers an abstraction called resilient distributed datasets (RDDs) to support these applications efficiently.

In above sentence, resilient distributed datasets (RDDs) also has broken link to RDD paper, I'll submit a PR in spark-website repo.

[SparkQA]

Test build #65511 has finished for PR 15121 at commit e2d40bd.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #65513 has finished for PR 15121 at commit 88960ef.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

Merged to master/2.0