PsychImaging('EnableNative10BitFramebuffer'): Change brackets for code path selection.

A bracket was misplaced, which could have caused the Windows-PTB to
take the wrong branch here, which by itself wouldn’t have made much
difference, just the resulting error message would have been more
confusing when trying to enable 10 npc on a Radeon card under Windows.
-> Should not have any effect on OSX or Linux.