Update apiref.rst
Update apiref.rst

fix the documentation about json_dumps. refs #471
markalanj/jansson,bstarynk/jansson,akheron/jansson,akheron/jansson,Mephistophiles/jansson,bstarynk/jansson,markalanj/jansson,markalanj/jansson,Mephistophiles/jansson,bstarynk/jansson,akheron/jansson