docs: ftrace: Fix small notation mistake

docs: ftrace: Fix small notation mistake

The use of iff ("if and only if") notation is not accurate in this case.

Suggested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Link: https://lore.kernel.org/r/22f9a98a972c3155c7b478247a087a5efafde774.1577231751.git.frank@generalsoftwareinc.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>