fix JsonWriterPipeline example

docs/topics/item-pipeline.rst

@@ -106,9 +106,12 @@ format::
 
    class JsonWriterPipeline(object):
 
-       def __init__(self):
+       def open_spider(self, spider):
            self.file = open('items.jl', 'wb')
 
+       def close_spider(self, spider):
+           self.file.close()
+
        def process_item(self, item, spider):
            line = json.dumps(dict(item)) + "\n"
            self.file.write(line)