doc: Fix a parameter of NotificationPublisher

The 'binary' parameter has been changed to the 'source'
since I95b5b0826190d396efe7bfc017f6081a6356da65.
But the notification document has not been updated yet.

Replace the 'binary' parameter with the 'source' parameter.

Change-Id: I141c90ac27d16f2e9c033bcd2f95ac08904a2f52
Closes-Bug: #1836005
(cherry picked from commit de31466)

Author: natsumetakashi

doc/source/reference/notifications.rst

@@ -290,7 +290,7 @@ payload.
 
 Note that the notification's publisher instance can be created two different
 ways. It can be created by instantiating the ``NotificationPublisher`` object
-with a ``host`` and a ``binary`` string parameter or it can be generated from a
+with a ``host`` and a ``source`` string parameter or it can be generated from a
 ``Service`` object by calling ``NotificationPublisher.from_service_obj``
 function.