Added required arguments to DecimalField model example (#1376)

docs/guide/usage.txt

@@ -20,7 +20,7 @@ Let's start with our model::
 
     class Product(models.Model):
         name = models.CharField(max_length=255)
-        price = models.DecimalField()
+        price = models.DecimalField(max_digits=5, decimal_places=2)
         description = models.TextField()
         release_date = models.DateField()
         manufacturer = models.ForeignKey(Manufacturer)