[MINOR][SPARKR] Add run command comment in examples

## What changes were proposed in this pull request?

There are two examples in r folder missing the run commands.

In this PR, I just add the missing comment, which is consistent with other examples.

## How was this patch tested?

Manual test.

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

Closes #17474 from wangmiao1981/stat.

examples/src/main/r/RSparkSQLExample.R

@@ -15,6 +15,9 @@
 # limitations under the License.
 #
 
+# To run this example use
+# ./bin/spark-submit examples/src/main/r/RSparkSQLExample.R
+
 library(SparkR)
 
 # $example on:init_session$

examples/src/main/r/dataframe.R

@@ -15,6 +15,9 @@
 # limitations under the License.
 #
 
+# To run this example use
+# ./bin/spark-submit examples/src/main/r/dataframe.R
+
 library(SparkR)
 
 # Initialize SparkSession