selftests: ftrace: fix typo in test description

The typo in the description shows up in test logs and output.
This patch submission is part of my application to the Linux Foundation
mentorship program: Linux kernel Bug Fixing Spring Unpaid 2024.

Signed-off-by: Ali Zahraee <ahzahraee@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>