Fix typo in comment of transformJsonAggConstructor()

An oversight of 3a8a1f3254.

Reported-by: Tender Wang <tndrwang@gmail.com>
Author: Tender Wang <tndrwang@gmail.com>
Backpatch-through: 16

src/backend/parser/parse_expr.c

@@ -3751,7 +3751,7 @@ transformJsonAggConstructor(ParseState *pstate, JsonAggConstructor *agg_ctor,
 /*
  * Transform JSON_OBJECTAGG() aggregate function.
  *
- * JSON_OBJECT() is transformed into a JsonConstructorExpr node of type
+ * JSON_OBJECTAGG() is transformed into a JsonConstructorExpr node of type
  * JSCTOR_JSON_OBJECTAGG, which at runtime becomes a
  * json[b]_object_agg[_unique][_strict](agg->arg->key, agg->arg->value) call
  * depending on the output JSON format.  The result is coerced to the target