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

hgext/strip.py

@@ -1,6 +1,6 b''
-"""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 _