linux-next: ftrace/docs: Fix spelling typos in ftrace-users.rst

This patch corrects some spelling typo in ftrace-users.rst

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>