docs/git-gc: fix typo "--prune=all" to "--prune=now"

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-gc.txt

@@ -76,7 +76,7 @@ be performed as well.
 --prune=<date>::
 	Prune loose objects older than date (default is 2 weeks ago,
 	overridable by the config variable `gc.pruneExpire`).
-	--prune=all prunes loose objects regardless of their age and
+	--prune=now prunes loose objects regardless of their age and
 	increases the risk of corruption if another process is writing to
 	the repository concurrently; see "NOTES" below. --prune is on by
 	default.