[SPARK-10663] Removed unnecessary invocation of DataFrame.toDF method.

The Scala example under the "Example: Pipeline" heading in this
document initializes the "test" variable to a DataFrame. Because test
is already a DF, there is not need to call test.toDF as the example
does in a subsequent line: model.transform(test.toDF). So, I removed
the extraneous toDF invocation.

Author: Matt Hagen <anonz3000@gmail.com>

Closes #8875 from hagenhaus/SPARK-10663.

docs/ml-guide.md

@@ -475,7 +475,7 @@ val test = sqlContext.createDataFrame(Seq(
 )).toDF("id", "text")
 
 // Make predictions on test documents.
-model.transform(test.toDF)
+model.transform(test)
   .select("id", "text", "probability", "prediction")
   .collect()
   .foreach { case Row(id: Long, text: String, prob: Vector, prediction: Double) =>