selftests: net: change shebang to bash in amt.sh

amt.sh is written in bash, not sh.
So, shebang should be bash.
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Acked-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/amt.sh

-#!/bin/sh
+#!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
 # Author: Taehee Yoo <ap420073@gmail.com>