Fix minor documentation issues

* author.txt was renamed to AUTHOR
* use git shortlog instead of git-shortlog
  because the latter is not necessarily in PATH

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>

doc/AUTHOR

 Include "gitstats" in the subject or prepare to battle the spam filters.
 
 See the following command for list of authors who have contributed:
-  $ git-shortlog HEAD
+  $ git shortlog HEAD
 
 Also thanks to the following people:
 Alexander Botero-Lowry

gitstats

 #!/usr/bin/env python
-# Copyright (c) 2007-2013 Heikki Hokkanen <hoxu@users.sf.net> & others (see doc/author.txt)
+# Copyright (c) 2007-2013 Heikki Hokkanen <hoxu@users.sf.net> & others (see doc/AUTHOR)
 # GPLv2 / GPLv3
 import datetime
 import getopt