[SPARK-11581][DOCS] Example mllib code in documentation incorrectly computes MSE

Author: Bharat Lal <bharat.iisc@gmail.com>

Closes #9560 from bharatl/SPARK-11581.

docs/mllib-decision-tree.md

@@ -439,7 +439,7 @@ Double testMSE =
     public Double call(Double a, Double b) {
       return a + b;
     }
-  }) / data.count();
+  }) / testData.count();
 System.out.println("Test Mean Squared Error: " + testMSE);
 System.out.println("Learned regression tree model:\n" + model.toDebugString());