Append TODO comment with color and opacity info

This class was not working, so could not test it. As there was already a
todo comment I simply added to the comment.

modules/spacecraftinstruments/rendering/renderablecrawlingline.cpp

@@ -49,8 +49,9 @@ namespace {
     };
 } // namespace
 
-// @TODO:  This clas is not properly working anymore and needs to be substantially
-//         rewritten
+// @TODO:  This class is not properly working anymore and needs to be substantially
+//         rewritten. When doing so, make sure that any color property uses three
+//         values, not four. The opacity should be handled separately
 
 namespace openspace {