[DataFrame] [Typo] Fix the typo

Author: Cheng Hao <hao.cheng@intel.com>

Closes #4717 from chenghao-intel/typo1 and squashes the following commits:

858d7b0 [Cheng Hao] update the typo

sql/core/src/main/scala/org/apache/spark/sql/DataFrame.scala

@@ -138,7 +138,7 @@ class DataFrame protected[sql](
 
   /**
    * An implicit conversion function internal to this class for us to avoid doing
-   * "new DataFrameImpl(...)" everywhere.
+   * "new DataFrame(...)" everywhere.
    */
   @inline private implicit def logicalPlanToDataFrame(logicalPlan: LogicalPlan): DataFrame = {
     new DataFrame(sqlContext, logicalPlan)