Fix typo

Author: MasloMaslane

oioioi/contests/controllers.py

@@ -992,7 +992,7 @@ def render_score_diff(self, diff):
             return '<span class="text-secondary">0</span>'
         if diff.to_int() > 0:
             return f'<span class="text-success">+{diff.to_int()}</span>'
-        return f'<span class="text-danger">{diff.to_int()}</span>'
+        return f'<span class="text-danger">-{diff.to_int()}</span>'
 
 
 class PastRoundsHiddenContestControllerMixin(object):