[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.

(cherry picked from commit a87471c8)
Signed-off-by: Herman van Hovell <hvanhovell@databricks.com>

sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSinkSuite.scala

@@ -152,7 +152,7 @@ class FileStreamSinkSuite extends StreamTest {
   }
 
   test("FileStreamSink - json") {
-    testFormat(Some("text"))
+    testFormat(Some("json"))
   }
 
   def testFormat(format: Option[String]): Unit = {