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

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.

Author: Xin Ren <iamshrek@126.com>

Closes #15121 from keypointt/SPARK-17567.

(cherry picked from commit f15d41be3ce7569736ccbf2ffe1bec265865f55d)
Signed-off-by: Sean Owen <291c18f3fb7528c712d9098b0e50a515ea0b91d5@cloudera.com>

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