gitk: Fix copyright symbol in About box message

Somehow it got corrupted in commit d93f171 ("gitk: Use themed tk
widgets").

Signed-off-by: Paul Mackerras <paulus@samba.org>

Author: paulusmack

gitk

@@ -2782,7 +2782,7 @@ proc about {} {
     message $w.m -text [mc "
 Gitk - a commit viewer for git
 
-Copyright \u00a9 2005-2009 Paul Mackerras
+Copyright © 2005-2009 Paul Mackerras
 
 Use and redistribute under the terms of the GNU General Public License"] \
 	    -justify center -aspect 400 -border 2 -bg white -relief groove