Fix a typo (thanks, Matt)

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/SubsurfaceScattering/SubsurfaceScattering.compute

     #if SSS_USE_TANGENT_PLANE
         float3 relPosVS   = vec.x * tangentX + vec.y * tangentY;
         float3 positionVS = centerPosVS + relPosVS;
-        float2 positionNDC = ComputeNormalizedDeviceCoordinates(positionCS, projMatrix);
+        float2 positionNDC = ComputeNormalizedDeviceCoordinates(positionVS, projMatrix);
 
         position = (int2)(positionNDC * _ScreenSize.xy);
         xy2      = dot(relPosVS.xy, relPosVS.xy);