gitweb: Fix not marking signoff lines in "log" view

The CSS selector for signoff lines style was too strict: in the "log"
view the commit message is not encompassed in container "page_body"
div.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

gitweb/gitweb.css

@@ -107,7 +107,7 @@ span.age {
 	font-style: italic;
 }
 
-div.page_body span.signoff {
+span.signoff {
 	color: #888888;
 }