fix ModelSerializer useage

cause of:
from snippets.models import Snippet

docs/tutorial/1-serialization.md

@@ -194,7 +194,7 @@ Open the file `snippets/serializers.py` again, and edit the `SnippetSerializer`
 
     class SnippetSerializer(serializers.ModelSerializer):
         class Meta:
-            model = models.Snippet
+            model = Snippet
             fields = ('pk', 'title', 'code', 'linenos', 'language', 'style')