Documentation: bootconfig: fix Sphinx block warning

Fix Sphinx format warning:

lnx-56-rc1/Documentation/admin-guide/bootconfig.rst:26: WARNING: Literal block expected; none found.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/07b3e31f-9b1e-1876-aa60-4436e4dd6da0@infradead.org


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