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) <[e-mail]>
Signed-off-by: Frank A. Cancio Bello <[e-mail]>
Reviewed-by: Steven Rostedt (VMware) <[e-mail]>
Reviewed-by: Joel Fernandes (Google) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/trace/ring-buffer-design.txt

···
 
 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