Fix Tutorial5 serializers.py typo

[Issue #N/A]

A slight typo at Tutorial5 causing an Exception "name 'models' is
not defined". A subtle one that can confuse Python/Django noobs
like me.

docs/tutorial/5-relationships-and-hyperlinked-apis.md

@@ -80,7 +80,7 @@ We can easily re-write our existing serializers to use hyperlinking.
         highlight = serializers.HyperlinkedIdentityField(view_name='snippet-highlight', format='html')
     
         class Meta:
-            model = models.Snippet
+            model = Snippet
             fields = ('url', 'highlight', 'owner',
                       'title', 'code', 'linenos', 'language', 'style')