docs: format kernel-parameters -- as code

The current ReStructuredText formatting results in "--", used to
indicate the end of the kernel command-line parameters, appearing as
an en-dash instead of two hyphens; this patch formats them as code,
"``--``", as done elsewhere in the documentation.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>