Fixed #20543 - Typo in db model fields docs.

Thanks i.amber.jain@ for the report.

docs/topics/db/models.txt

@@ -182,7 +182,7 @@ ones:
                 ('L', 'Large'),
             )
             name = models.CharField(max_length=60)
-            shirt_size = models.CharField(max_length=2, choices=SHIRT_SIZES)
+            shirt_size = models.CharField(max_length=1, choices=SHIRT_SIZES)
 
     ::