docs: fix gst-validate-launch test suite example.
According to gst-validate-1.0 --list-scenarios, play_5s is not a valid scenario. Runnning the test suite example with it ends up raising an AttributeError. Switching to play_15s makes it work.
According to gst-validate-1.0 --list-scenarios, play_5s is not a valid scenario. Runnning the test suite example with it ends up raising an AttributeError. Switching to play_15s makes it work.