Headings of Sage library code files should state GPLv2+ instead of GPL without version

[jdemeyer]

In the developer documentation, in the section ​Headings of Sage library code files, there is an example heading stating

#*****************************************************************************
#       Copyright (C) 2008 YOUR NAME <your email>
#
#  Distributed under the terms of the GNU General Public License (GPL)
#                  http://www.gnu.org/licenses/
#*****************************************************************************

I think this should be changed to state the version of the GPL. The current wording is maybe not technically wrong, but adding the version would certainly clarify things.