tools: testing: ftrace: trigger: fix spelling mistake

Fix typo: "tigger" --> "trigger"

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>