tracing/doc: Fix typos in histogram-design.rst

There's a few typos in the histogram-design.rst document that need need to
be fixed.

Cc: Tom Zanussi <zanussi@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>