updateCredits: Fix collation name

updateCredits: Fix collation name

'uca-default-u-kn' is a valid name for MediaWiki's category collation
functionality ($wgCategoryCollation / Collation class), not for PHP's
collation functionality (Collator class).

'uca-default-u-kn' for Collation maps to 'root-u-kn' for Collator
(see Collation::factory()), and the '-u-kn' suffix is later stripped
to enable numeric collation (see IcuCollation::_construct()).

This doesn't seem to result in any changes in the output.

Change-Id: I8cc7d283aee5408f8b7bed365661ff286236a694