Fixed doc example

Image.data is a method, not an attribute.

DocImpact

Change-Id: I16555e456dcb2c9719de6c44c01ab68e82ad0590

doc/source/index.rst

@@ -11,7 +11,7 @@ In order to use the python api directly, you must first obtain an auth token and
     >>> print image.status
     'active'
     >>> with open('/tmp/copyimage.iso', 'wb') as f:
-            for chunk in image.data:
+            for chunk in image.data():
                 f.write(chunk)
     >>> image.delete()