tracing/histogram: Fix stacktrace histogram Documententation

Fix a small problem with the histogram specification in the
Documentation, and change the example to show output using a
stacktrace field rather than the global stacktrace.

Link: https://lkml.kernel.org/r/f75f807dd4998249e513515f703a2ff7407605f4.1676063532.git.zanussi@kernel.org



Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>