Fix variable usage example in docs (#3261)
Fix variable usage example in docs (#3261)

I found this while reading through the image output section and it seems wrong given the following examples.
OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio