rebase: fix saving of --signoff state for am-based rebases

This was an error introduced in the conversion from shell in commit
21853626eac5 ("built-in rebase: call `git am` directly", 2019-01-18),
which was noticed by a random browsing of the code.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/rebase.c b/builtin/rebase.c
index 4a20582..86f869c 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -685,7 +685,7 @@
 		write_file(state_dir_path("gpg_sign_opt", opts), "%s",
 			   opts->gpg_sign_opt);
 	if (opts->signoff)
-		write_file(state_dir_path("strategy", opts), "--signoff");
+		write_file(state_dir_path("signoff", opts), "--signoff");
 
 	return 0;
 }