strip: bring extension description in line with style and copy-edit

strip: bring extension description in line with style and copy-edit

--- a/hgext/strip.py	Thu Sep 26 11:11:39 2013 +0200
+++ b/hgext/strip.py	Fri Sep 27 21:54:53 2013 -0500
@@ -1,6 +1,6 @@
-"""This extension contains the strip commands.
+"""strip changesets and their descendents from history
 
-This extensions allows to strip changesets and all their descendants from the
+This extension allows to strip changesets and all their descendants from the
 repository. See the command help for details.
 """
 from mercurial.i18n import _