Documentation/config: fix stale "git prune --worktree" reference

This should have been changed to "git worktree prune" by df0b6cf
(worktree: new place for "git prune --worktrees", 2015-06-29)

[es: reword commit message]

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1231,7 +1231,7 @@ gc.pruneexpire::
 
 gc.pruneworktreesexpire::
 	When 'git gc' is run, it will call
-	'prune --worktrees --expire 3.months.ago'.
+	'git worktree prune --expire 3.months.ago'.
 	Override the grace period with this config variable. The value
 	"now" may be used to disable the grace period and prune
 	$GIT_DIR/worktrees immediately.