[ticket/12901] Fix type hinting of style $level

PHPBB3-12901
[ticket/12901] Fix another wrong type hint for $level

PHPBB3-12901
Merge pull request #2799 from nickvergessen/ticket/12901

Ticket/12901 Wrong type hint in show_available_child_styles() doc block
Merge branch 'develop-ascraeus' into develop