Fix PublicContrib.is_public() behavior

Was doing the oposite of what is supposed !

wifiwithme/apps/contribmap/models.py

@@ -134,7 +134,7 @@ class Contrib(models.Model):
         return merge_intervals(angles)
 
     def is_public(self):
-        return not self.privacy_coordinates
+        return self.privacy_coordinates
 
     def _may_be_public(self, field):
         return field in self.PUBLIC_FIELDS