docs(shaders): update comment

shaders/grayscale.glsl.mustache

@@ -16,7 +16,7 @@ const int AVERAGE = 2;
  */
 const int Type = {{#nc}}{{type}} ? LUMINOSITY{{/nc}};
 
-// Enum for luma coefficients
+// Enum for selecting luma coefficients
 const int PAL = 0;
 const int HDTV = 1;
 const int HDR = 2;