hooklib: update documentation of changeset_obsoletedfor for changed hook type

This updates usage example of changeset_obsoleted to reflect the move from
pretxnclose to txnclose made in 04ef381000a8 (hooklib: fix detection of
successors for changeset_obsoleted).

Differential Revision: https://phab.mercurial-scm.org/D8929

hgext/hooklib/changeset_obsoleted.py

@@ -13,7 +13,7 b' Usage:'
   messageidseed = myseed
 
   [hooks]
-  pretxnclose.changeset_obsoleted = \
+  txnclose.changeset_obsoleted = \
     python:hgext.hooklib.changeset_obsoleted.hook
 """