[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.