bootconfig/tracing/ktest: Update ktest example for boot-time tracing

Update ktest example for the boot-time tracing with histogram
options. Note that since the histogram option uses "trace()" action
instead of "EVENT()", this updates the matching pattern too.

Link: https://lkml.kernel.org/r/162856130208.203126.4458319094852152589.stgit@devnote2



Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>