Fixed typos in StringAgg and JSONBAgg examples

docs/ref/contrib/postgres/aggregates.txt

@@ -157,7 +157,7 @@ General-purpose aggregation functions
         class Room(models.Model):
             number = models.IntegerField(unique=True)
 
-        class HotelReservation(model.Model):
+        class HotelReservation(models.Model):
             room = models.ForeignKey('Room', on_delete=models.CASCADE)
             start = models.DateTimeField()
             end = models.DateTimeField()
@@ -213,7 +213,7 @@ General-purpose aggregation functions
         class Publication(models.Model):
             title = models.CharField(max_length=30)
 
-        class Article(model.Model):
+        class Article(models.Model):
             headline = models.CharField(max_length=100)
             publications = models.ManyToManyField(Publication)
 
@@ -231,7 +231,7 @@ General-purpose aggregation functions
         ... ).values('headline', 'publication_names')
         <QuerySet [{
             'headline': 'NASA uses Python', 'publication_names': 'Science News, The Python Journal'
-        ]}]>
+        }]>
 
     .. deprecated:: 4.0