added fact from exercise sheet

inputs/lecture_08.tex

@@ -146,3 +146,8 @@ for any $k \in \N$.
     \]
     hence $\bP[T] \in \{0,1\}$.
 \end{refproof}
+\begin{fact}+[Exercise 5.2 (b)]
+    Any random variable measurable with respect to a $\bP$-trivial
+    $\sigma$-algebra is a.s.~a constant.
+\end{fact}
+%\todo{copy proof}