improve 'date +%C' documentation

* src/date.c (usage): Use 20, not 21, for current century.
* THANKS: Update.
Reported by Dameon G. Rogers, fix suggested by Philip Rowlands.

Signed-off-by: Eric Blake <ebb9@byu.net>

THANKS

@@ -110,6 +110,7 @@ Cray-Cyber Project                  http://www.cray-cyber.org
 Cristian Cadar                      cristic@stanford.edu
 Cyril Bouthors                      cyril@bouthors.org
 Dale Scheetz                        dwarf@polaris.net
+Dameon G. Rogers                    dgr03@uark.edu
 Dan Hagerty                         hag@gnu.ai.it.edu
 Dan Jacobson                        http://www.geocities.com/jidani
 Dan Pascu                           dan@services.iiruc.ro
@@ -434,6 +435,7 @@ Peter Seebach                       seebs@taniemarie.solon.com
 Petter Reinholdtsen                 pere@hungry.com
 Phelippe Neveu                      pneveu@pcigeomatics.com
 Phil Richards                       phil.richards@vf.vodafone.co.uk
+Philip Rowlands                     phr@doc.ic.ac.uk
 Philippe De Muyter                  phdm@macqel.be
 Philippe Schnoebelen                Philippe.Schnoebelen@imag.fr
 Phillip Jones                       mouse@datastacks.com

src/date.c

@@ -166,7 +166,7 @@ specifies Coordinated Universal Time.  Interpreted sequences are:\n\
   %c   locale's date and time (e.g., Thu Mar  3 23:05:25 2005)\n\
 "), stdout);
       fputs (_("\
-  %C   century; like %Y, except omit last two digits (e.g., 21)\n\
+  %C   century; like %Y, except omit last two digits (e.g., 20)\n\
   %d   day of month (e.g, 01)\n\
   %D   date; same as %m/%d/%y\n\
   %e   day of month, space padded; same as %_d\n\