[S 21.5] Fix signs in time zone string.

Marcus Crestani writes:
 > SUPERSEDES PATCH 21.5
 > 
 > >>>>>"SJT" == Stephen J Turnbull <[email protected]> writes:
 > SJT> "+0" is prettier, but there may be code that looks for "-0" explicitly
 > SJT> out there (eg, to test for GMT).  If you're going to do it differently
 > SJT> from GNU, please document it.
 > 
 > Sure, see below.  Is the comment clear enough?  Should it mention that
 > GNU Emacs does it differently?

I'm sorry, I meant in the docstring, but upon looking more closely
that makes no sense since in fact this string never is seen by a user
as far as I can tell.  If that's true, the comment is great!

Steve