[SPARK-16040][MLLIB][DOC] spark.mllib PIC document extra line of refernece

## What changes were proposed in this pull request?

In the 2.0 document, Line "A full example that produces the experiment described in the PIC paper can be found under examples/." is redundant.

There is already "Find full example code at "examples/src/main/scala/org/apache/spark/examples/mllib/PowerIterationClusteringExample.scala" in the Spark repo.".

We should remove the first line, which is consistent with other documents.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

Manual test

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #13755 from wangmiao1981/doc.

docs/mllib-clustering.md

@@ -170,10 +170,6 @@ which contains the computed clustering assignments.
 Refer to the [`PowerIterationClustering` Scala docs](api/scala/index.html#org.apache.spark.mllib.clustering.PowerIterationClustering) and [`PowerIterationClusteringModel` Scala docs](api/scala/index.html#org.apache.spark.mllib.clustering.PowerIterationClusteringModel) for details on the API.
 
 {% include_example scala/org/apache/spark/examples/mllib/PowerIterationClusteringExample.scala %}
-
-A full example that produces the experiment described in the PIC paper can be found under
-[`examples/`](https://github.com/apache/spark/blob/master/examples/src/main/scala/org/apache/spark/examples/mllib/PowerIterationClusteringExample.scala).
-
 </div>
 
 <div data-lang="java" markdown="1">