Update preferential-gpu-scheduling.md (#7144)
Update preferential-gpu-scheduling.md (#7144)

Nasty typo `DeclinedScareResources` instead `DeclinedScarceResources`, so this documentation is not properly indexed on Google and did not showed up after job seeking for information being stuck thanks to the new marathon 1.8+ gpu_scheduling_behavior set to "restricted" as default.
mesosphere/marathon,mesosphere/marathon,mesosphere/marathon,mesosphere/marathon,mesosphere/marathon