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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/ring-buffer-design.txt

@@ -37,7 +37,7 @@ commit_page - a pointer to the page with the last finished non-nested write.
 
 cmpxchg - hardware-assisted atomic transaction that performs the following:
 
-   A = B iff previous A == C
+   A = B if previous A == C
 
    R = cmpxchg(A, C, B) is saying that we replace A with B if and only if
       current A is equal to C, and we put the old (current) A into R