Cleanup: Fix comment

Enum used for separating individual layer, not just active layer

Pull Request: https://projects.blender.org/blender/blender/pulls/135771

source/blender/editors/grease_pencil/intern/grease_pencil_edit.cc

@@ -2086,7 +2086,7 @@ enum class SeparateMode : int8_t {
   SELECTED = 0,
   /* By Material. */
   MATERIAL = 1,
-  /* By Active Layer. */
+  /* By each Layer. */
   LAYER = 2,
 };