Change slideshow_by_tag into slideshows_by_tag

Up to know there was an API cal to slideshow_by_tag which not exists
in slideshare APIv2. According to the documentation there is however
slideshows_by_tag:
http://www.slideshare.net/developers/documentation

The slideshow_by_tag is still available in the APIv1:
http://www.slideshare.net/developers/documentation/v1