docs/git-gc: fix default value for `--aggressiveDepth`

In commit 07e7dbf0d (gc: default aggressive depth to 50, 2016-08-11),
the default aggressive depth of git-gc has been changed to 50. While
git-config(1) has been updated to represent the new default value,
git-gc(1) still mentions the old value. This patch fixes it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-gc.txt

@@ -127,7 +127,7 @@ the documentation for the --window' option in linkgit:git-repack[1] for
 more details.  This defaults to 250.
 
 Similarly, the optional configuration variable `gc.aggressiveDepth`
-controls --depth option in linkgit:git-repack[1]. This defaults to 250.
+controls --depth option in linkgit:git-repack[1]. This defaults to 50.
 
 The optional configuration variable `gc.pruneExpire` controls how old
 the unreferenced loose objects have to be before they are pruned.  The