Fix typo in the New Procedures section of NV_shading_rate_image. (#378)

The old version of the spec created a prototype for
ShadingRateSampleOrderNV by copy-pasting the definition for
ShadingRateImageBarrierNV.  Unfortunately, that copy-paste failed to
fix the function name, so we have two instances of
ShadingRateImageBarrierNV.
1 file changed