HDRenderPipeline: Fix issue with transmittance being a float and not a float3

ScriptableRenderPipeline/Core/ShaderLibrary/VolumeRendering.hlsl

 }
 
 // Absorption coefficient from Disney: http://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf
-float TransmittanceColorAtDistanceToAbsorption(float3 transmittanceColor, float atDistance)
+float3 TransmittanceColorAtDistanceToAbsorption(float3 transmittanceColor, float atDistance)
 {
     return -log(transmittanceColor + 0.00001) / max(atDistance, 0.000001);
 }