doc: fix careless document miss in help of hgext/notify

according to configuration example below, and direction of changeset
transference, this paragraph should describe about "changegroup" hook.

  [hooks]
  # one email for each incoming changeset
  incoming.notify = python:hgext.notify.hook
  # one email for all incoming changesets
  changegroup.notify = python:hgext.notify.hook

  # one email for all outgoing changesets
  outgoing.notify = python:hgext.notify.hook

hgext/notify.py

@@ -11,7 +11,7 b' This extension let you run hooks sending'
 changesets are being pushed, from the sending or receiving side.
 
 First, enable the extension as explained in :hg:`help extensions`, and
-register the hook you want to run. ``incoming`` and ``outgoing`` hooks
+register the hook you want to run. ``incoming`` and ``changegroup`` hooks
 are run by the changesets receiver while the ``outgoing`` one is for
 the sender::
 

tests/test-notify.t

@@ -23,7 +23,7 b''
   are being pushed, from the sending or receiving side.
   
   First, enable the extension as explained in "hg help extensions", and register
-  the hook you want to run. "incoming" and "outgoing" hooks are run by the
+  the hook you want to run. "incoming" and "changegroup" hooks are run by the
   changesets receiver while the "outgoing" one is for the sender:
   
     [hooks]