selftest/ftrace: fix typo in ftracetest script

Fix a typo in ftracetest script which is run when running the kselftests
for ftrace.

s/faii/fail

Signed-off-by: Kousik Sanagavarapu <five231003@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>