docs: ftrace: Fix typos

Fix minor typos in the doc.

Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
Link: https://lore.kernel.org/r/9ef705d0208a4ca0852fed69bc0838a589a4df85.1577231751.git.frank@generalsoftwareinc.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>