tracing: Fix doc mistakes in trace sample

As the example below shows, DECLARE_EVENT_CLASS() is used instead of
DEFINE_EVENT_CLASS().

Link: http://lkml.kernel.org/r/20200428214959.11259-1-richard.weiyang@gmail.com



Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>