validate: Update for false-positive bt601
As of gst-plugins-base!724 (merged) , we write the correct transfer function for bt601. instead of the (functionally identical but different ISO code) transfer function for bt709. Files created using GStreamer prior to that commit write the wrong transfer function for bt601 and are, strictly speaking, 2:4:5:4 instead. This commit changes those expected values to 2:4:5:4.