Minor Fix: Typo fix at Nuttx wiki: NX Graphics Subsystem

[[e-mail] [nuttx]]

I found a minor typo error in NX Graphics Subsystem section, 2.2.2 nxgl_yuv2rgb() function that its comment needs to be modified from:
Description: Convert 8-bit RGB triplet to 8-bit YUV triplet.

to
Description: Convert 8-bit YUV triplet to 8-bit RGB triplet.

[[e-mail] [nuttx]]

Fixed... Thanks for you good eyes!