docs: ftrace-uses.rst fix varios code-block directives

ftrace-uses.rst is not yet included into any toctree, but since it is
a .rst file, it is parsed by the Sphinx build. Thats, why we see some
WARNINGS:

- trace/ftrace-uses.rst:53: WARNING: Definition list ends without a blank line; unexpected unindent.
- trace/ftrace-uses.rst:89: WARNING: Inline emphasis start-string without end-string.
- trace/ftrace-uses.rst:89: WARNING: Inline emphasis start-string without end-strin

Fixing the code-block directives results in a less noisy build, but the 'not
included' WARNING will be stay:

- trace/ftrace-uses.rst: WARNING: document isn't included in any toctree

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>