fix(Highlighter): fix syntax error with use of CombineMeshes

CombineMeshes can only be called on a mesh object and not on a
MeshFilter. This was throwing a critical error and has now been
fixed.

Assets/VRTK/Scripts/Interactions/Highlighters/VRTK_OutlineObjectCopyHighlighter.cs

@@ -179,7 +179,7 @@ private void CreateHighlightModel()
                     }
 
                     highlightMesh.mesh = new Mesh();
-                    highlightMesh.CombineMeshes(combine.ToArray(), true, false);
+                    highlightMesh.mesh.CombineMeshes(combine.ToArray(), true, false);
                 }
                 else
                 {