MAke Signals code example format correctly
MAke Signals code example format correctly

Single colon instead of double to introduce code section
bmihelac/django-import-export,copperleaftech/django-import-export,PetrDlouhy/django-import-export,jnns/django-import-export,django-import-export/django-import-export,jnns/django-import-export,django-import-export/django-import-export,bmihelac/django-import-export,daniell/django-import-export,jnns/django-import-export,bmihelac/django-import-export,jnns/django-import-export,django-import-export/django-import-export,copperleaftech/django-import-export,bmihelac/django-import-export,copperleaftech/django-import-export,daniell/django-import-export,daniell/django-import-export,copperleaftech/django-import-export,PetrDlouhy/django-import-export,daniell/django-import-export,django-import-export/django-import-export,PetrDlouhy/django-import-export,PetrDlouhy/django-import-export