selftest/ftrace: Fix typo in trigger-snapshot.tc

This patch fixes a spelling typo in trigger-snapshot.tc

[skhan@linuxfoundation.org: Fix typo in commit log]
Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>