Add missing `blank=True` to model in tutorial.

docs/tutorial/1-serialization.md

@@ -86,7 +86,7 @@ For the purposes of this tutorial we're going to start by creating a simple `Sni
     
     class Snippet(models.Model):
         created = models.DateTimeField(auto_now_add=True)
-        title = models.CharField(max_length=100, default='')
+        title = models.CharField(max_length=100, blank=True, default='')
         code = models.TextField()
         linenos = models.BooleanField(default=False)
         language = models.CharField(choices=LANGUAGE_CHOICES,