[MINOR][SQL][DOC] Fix to_json example in function description and doc

## What changes were proposed in this pull request?

This PR fixes the an example for `to_json` in doc and function description.

- http://spark.apache.org/docs/2.3.0/api/sql/#to_json
- `describe function extended`

## How was this patch tested?

Pass the Jenkins with the updated test.

Closes apache#22096 from dongjoon-hyun/minor_json.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/jsonExpressions.scala

@@ -625,7 +625,7 @@ case class JsonToStructs(
        {"a":1,"b":2}
       > SELECT _FUNC_(named_struct('time', to_timestamp('2015-08-26', 'yyyy-MM-dd')), map('timestampFormat', 'dd/MM/yyyy'));
        {"time":"26/08/2015"}
-      > SELECT _FUNC_(array(named_struct('a', 1, 'b', 2));
+      > SELECT _FUNC_(array(named_struct('a', 1, 'b', 2)));
        [{"a":1,"b":2}]
       > SELECT _FUNC_(map('a', named_struct('b', 1)));
        {"a":{"b":1}}

sql/core/src/test/resources/sql-tests/results/json-functions.sql.out

@@ -24,7 +24,7 @@ Extended Usage:
        {"a":1,"b":2}
       > SELECT to_json(named_struct('time', to_timestamp('2015-08-26', 'yyyy-MM-dd')), map('timestampFormat', 'dd/MM/yyyy'));
        {"time":"26/08/2015"}
-      > SELECT to_json(array(named_struct('a', 1, 'b', 2));
+      > SELECT to_json(array(named_struct('a', 1, 'b', 2)));
        [{"a":1,"b":2}]
       > SELECT to_json(map('a', named_struct('b', 1)));
        {"a":{"b":1}}