message: Fix typo in export_savedmodel api doc.
diff: @@ -486,7 +486,7 @@ class Estimator(object):
  `ExportOutput`s, and the inputs are always the input receivers provided by
  the serving_input_receiver_fn.
  - Extra assets may be written into the SavedModel via the extra_assets
  + Extra assets may be written into the SavedModel via the assets_extra
  argument. This should be a dict, where each key gives a destination path
  (including the filename) relative to the assets.extra directory. The
  corresponding value gives the full path of the source file to be copied.
  