[SPARK-18192][MINOR][FOLLOWUP] Missed json test in FileStreamSinkSuite

## What changes were proposed in this pull request?

This PR proposes to fix

```diff
 test("FileStreamSink - json") {
-  testFormat(Some("text"))
+  testFormat(Some("json"))
 }
```

`text` is being tested above

```
test("FileStreamSink - text") {
  testFormat(Some("text"))
}
```

## How was this patch tested?

Fixed test in `FileStreamSinkSuite.scala`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #15785 from HyukjinKwon/SPARK-18192.