t3400: stop referring to the scripted rebase

One test case's title mentioned the then-current implementation detail
that the `--am` backend was implemented in `git-rebase--am.sh`.

This is no longer the case, so let's update the title to reflect the
current reality.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3400-rebase.sh

@@ -285,7 +285,7 @@ EOF
 	test_cmp From_.msg out
 '
 
-test_expect_success 'rebase--am.sh and --show-current-patch' '
+test_expect_success 'rebase --am and --show-current-patch' '
 	test_create_repo conflict-apply &&
 	(
 		cd conflict-apply &&