Update challenges.jade

Additional changes in translations required before merging!

https://trello.com/c/SvTsLdRF/12-translations
"0leg
It would be great if we could have "%number_of% Participants" instead of just "Participants" in the source language so that we could translate it as "участников: %number_of%" which is the only correct way to translate it into Russian without grammatical mistakes. (This example is from Challenges tab.)"

website/views/options/social/challenges.jade

@@ -152,8 +152,7 @@ script(type='text/ng-template', id='partials/options.social.challenges.html')
                   =env.t('by') + ' '
                   a(ng-click='clickMember(challenge.leader._id, true)') {{challenge.leader.profile.name}}
                 li
-                  | {{challenge.memberCount}}
-                  = ' ' + env.t('participants')
+                  =env.t('participants', {membercount: "{{challenge.memberCount}}"})
                 li(ng-show='challenge.prize')
                   p
                     // prize