Bug 28943: (QA follow-up) Fix opac.pref

Bug 28943: (QA follow-up) Fix opac.pref

Fix YAML syntax of opac.pref, we must use dash for new lines.

Also removes duplicate text before syspref.
No need to say "code" since it is now a list of categories.

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 31fffa6006bf25bfa303fa44f4ae56f461cebb59)

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>