update assignment with feedback from students

assignments_2021/assignment1_path_tracing/assignment.tex

@@ -139,7 +139,8 @@ If the closest intersected object is a light, compute its contribution using the
 This should only require a small edit from the \texttt{ao} integrator.
 
 \subsection{Light surface sampling (up to 6 points)}
-Light surface sampling is important for performant path tracers (it's referenced as "next event estimation" or "direct light sampling" there).
+Light surface sampling is important for performant path tracers.
+It's referenced as "next event estimation" or "direct light sampling" there (you will have the chance to implement it in a later assignment).
 In contrast to hemisphere sampling, you are not simply shooting rays around the hemisphere and hope to find light.
 Instead, you try to connect hit points directly to light sources and check if that connection is possible.
 If you implement it, you should see improvements immediately.