tracing: Fix synth_event_add_val() kernel-doc comment

It's named field here.

Link: https://lkml.kernel.org/r/20210516022410.64271-1-hqjagain@gmail.com



Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>