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>

samples/trace_events/trace-events-sample.h

@@ -416,7 +416,7 @@ TRACE_EVENT_FN(foo_bar_with_fn,
  * Note, TRACE_EVENT() itself is simply defined as:
  *
  * #define TRACE_EVENT(name, proto, args, tstruct, assign, printk)  \
- *  DEFINE_EVENT_CLASS(name, proto, args, tstruct, assign, printk); \
+ *  DECLARE_EVENT_CLASS(name, proto, args, tstruct, assign, printk); \
  *  DEFINE_EVENT(name, name, proto, args)
  *
  * The DEFINE_EVENT() also can be declared with conditions and reg functions: