docs: boothd.8: fix runaway formatting for what's config file snippet

This was (likely) exposed with asciidoctor being updated to v2.0.0+
resulting in this "visual" (as in actually viewing the man page) diff
before vs. after:

• ticket="manual-ticket"
• [...]
• mode = manual
• [...]

+ ticket="manual-ticket" [...] mode = manual [...]

Marking that part as a literal block (in an established manner)
restores the sanity.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>