Bug 29589: MARCOverlayRules - fix translatability issue

Bug 29589: MARCOverlayRules - fix translatability issue

Strings must not contain html tags, we should use KohaSpan instead.

Test plan:
Apply the patch, update translation for a given language, edit XX-YY-staff-prog.po

Notice that the string is now
"The %s preference is not set, don't forget to enable it for rules to take effect."

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>