[MINOR][SPARKR] update sparkR DataFrame.R comment
[MINOR][SPARKR] update sparkR DataFrame.R comment

## What changes were proposed in this pull request?

update sparkR DataFrame.R comment
SQLContext ==> SparkSession

## How was this patch tested?

N/A

Author: WeichenXu <WeichenXu123@outlook.com>

Closes #13946 from WeichenXu123/sparkR_comment_update_sparkSession.

(cherry picked from commit d59ba8e30751bbf91d49f5530b8242a12bbfb569)
Signed-off-by: Shivaram Venkataraman <ad63b47dfac1951bde2b661ba9fbbe123f686bee@cs.berkeley.edu>

gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark