Use the correct i18n helper for namespaced string

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/views/projects/tags/index.html.haml

 - @no_container = true
 - @sort ||= sort_value_recently_updated
-- page_title _('TagsPage|Tags')
+- page_title s_('TagsPage|Tags')
 - add_to_breadcrumbs("Repository", project_tree_path(@project))
 
 .flex-list{ class: container_class }