tracing: Fix inconsistent style of mini-HOWTO

Each description should start with a hyphen and a space. Insert
spaces to fix it.

Link: https://lkml.kernel.org/r/TYCP286MB19130AA4A9C6FC5A8793DED2A1359@TYCP286MB1913.JPNP286.PROD.OUTLOOK.COM



Signed-off-by: Oscar Shiang <oscar0225@livemail.tw>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>