Documentation/config.txt: default gc.aggressiveWindow is 250, not 10

The default for gc.aggressiveWindow has been 250 since 1c192f34
(gc --aggressive: make it really aggressive, 2007-12-06).
Signed-off-by: NJay Soffian <jaysoffian@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -885,7 +885,7 @@ format.signoff::
 gc.aggressiveWindow::
 	The window size parameter used in the delta compression
 	algorithm used by 'git gc --aggressive'.  This defaults
-	to 10.
+	to 250.
 
 gc.auto::
 	When there are approximately more than this many loose