Documentation: tracing: Fix typos in boot-time tracing documentation

Fix typos in boottime-trace.rst according to Randy's suggestions.

Link: http://lkml.kernel.org/r/157949060335.25888.13153184562531693684.stgit@devnote2



Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>