[SPARK-14937][ML][DOCUMENT] spark.ml LogisticRegression sqlCtx in scala is inconsistent with java and python

## What changes were proposed in this pull request?
In spark.ml document, the LogisticRegression scala example uses sqlCtx. It is inconsistent with java and python examples which use sqlContext. In addition, a user can't copy & paste to run the example in spark-shell as sqlCtx doesn't exist in spark-shell while sqlContext exists.

Change the scala example referred by the spark.ml example.

## How was this patch tested?

Compile the example scala file and it passes compilation.

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

Closes #12717 from wangmiao1981/doc.