media-server: Use a hyphen instead of underscore in provider type
media-server: Use a hyphen instead of underscore in provider type

Fixes: https://bugzilla.gnome.org/736407

Distrotech/gnome-online-accounts,halfline/gnome-online-accounts,felipeborges/gnome-online-accounts,felipeborges/gnome-online-accounts,felipeborges/gnome-online-accounts,halfline/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,halfline/gnome-online-accounts,Distrotech/gnome-online-accounts,Distrotech/gnome-online-accounts,ritzk/gnome-online-accounts