message: Fix typo in MDL thin_surface
diff: <implementation name="IM_surface_genmdl" nodedef="ND_surface" target="genmdl" />
  <!-- <thin_surface> -->
  - <implementation name="IM_thin_surface_genmdl" nodedef="ND_thin_surface" sourcecode="mx::pbrlib::mx_thin_surface(mxp_front_bsdf:{{front_bsdf}}, mxp_front_edf:{{front_edf}}, (mxp_back_bsdf:{{back_bsdf}}, mxp_back_edf:{{back_edf}}, mxp_opacity:{{opacity}})" target="genmdl" />
  + <implementation name="IM_thin_surface_genmdl" nodedef="ND_thin_surface" sourcecode="mx::pbrlib::mx_thin_surface(mxp_front_bsdf:{{front_bsdf}}, mxp_front_edf:{{front_edf}}, mxp_back_bsdf:{{back_bsdf}}, mxp_back_edf:{{back_edf}}, mxp_opacity:{{opacity}})" target="genmdl" />
  <!-- <volume> -->
  <implementation name="IM_volume_genmdl" nodedef="ND_volume" sourcecode="mx::pbrlib::mx_volume(mxp_vdf:{{vdf}}, mxp_edf:{{edf}})" target="genmdl" />
  