[1.6.x] Corrected a usage example of `related_query_name`.

Thanks to Petr Glotov for spotting this.

Backport of 05a8cef428 from master

docs/ref/models/fields.txt

@@ -1110,7 +1110,7 @@ define the details of how the relation works.
             name = models.CharField(max_length=255)
 
         # That's now the name of the reverse filter
-        article_instance.filter(tag__name="important")
+        Article.objects.filter(tag__name="important")
 
 .. attribute:: ForeignKey.to_field