Documentation/git-gc.txt: default --aggressive window is 250, not 10

The default --aggressive window has been 250 since 1c192f34 "gc
--aggressive: make it really aggressive", released in git v1.6.3.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/git-gc.txt

@@ -106,7 +106,7 @@ much time is spent optimizing the delta compression of the objects in
 the repository when the --aggressive option is specified.  The larger
 the value, the more time is spent optimizing the delta compression.  See
 the documentation for the --window' option in linkgit:git-repack[1] for
-more details.  This defaults to 10.
+more details.  This defaults to 250.
 
 The optional configuration variable 'gc.pruneExpire' controls how old
 the unreferenced loose objects have to be before they are pruned.  The