Fix manifest description (regression from #4122) (#4324)

app/views/manifests/show.json.rabl

@@ -2,7 +2,7 @@ object false
 
 node(:name)             { Setting.site_title }
 node(:short_name)       { Setting.site_title }
-node(:description)      { strip_tags(Setting.site_description.presence || I18n.t('about.about_mastodon')) }
+node(:description)      { strip_tags(Setting.site_description.presence || I18n.t('about.about_mastodon_html')) }
 node(:icons)            { [{ src: '/android-chrome-192x192.png', sizes: '192x192', type: 'image/png' }] }
 node(:theme_color)      { '#282c37' }
 node(:background_color) { '#d9e1e8' }