[MINOR][SPARK SUBMIT] Print out R file usage in spark-submit

## What changes were proposed in this pull request?

Currently, running the shell below:

```bash
$ ./bin/spark-submit tmp.R a b c
```

with R file, `tmp.R` as below:

```r
#!/usr/bin/env Rscript

library(SparkR)
sparkRSQL.init(sparkR.init(master = "local"))
collect(createDataFrame(list(list(1))))
print(commandArgs(trailingOnly = TRUE))
```

working fine as below:

```bash
  _1
1  1
[1] "a" "b" "c"
```

However, it looks not printed in usage documentation as below:

```bash
$ ./bin/spark-submit
```

```
Usage: spark-submit [options] <app jar | python file> [app arguments]
...
```

For `./bin/sparkR`, it looks fine as below:

```bash
$ ./bin/sparkR tmp.R
```

```
Running R applications through 'sparkR' is not supported as of Spark 2.0.
Use ./bin/spark-submit <R file>
```

Running the script below:

```bash
$ ./bin/spark-submit
```

**Before**

```
Usage: spark-submit [options] <app jar | python file> [app arguments]
...
```

**After**

```
Usage: spark-submit [options] <app jar | python file | R file> [app arguments]
...
```

## How was this patch tested?

Manually tested.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18505 from HyukjinKwon/minor-doc-summit.