selftest: ftrace: fix minor typo in log

Resolves a spelling error in the test log, preventing potential
confusion.

Signed-off-by: Vincenzo Mezzela <vincenzo.mezzela@gmail.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>