Added Fubini's theorem.

src/measure/vector/variation.tex

@@ -117,7 +117,7 @@
         \mu(A) = \sum_{n = 1}^\infty \mu_n(A) = \sum_{n = 1}^\infty \sum_{k = 1}^\infty \mu_n(A_k) = \sum_{k = 1}^\infty \mu(A_k)
     \]
 
-    by Fubini's theorem.
+    by \hyperref[Fubini's theorem]{theorem:fubini-tonelli}.
     
     % TODO: Actually link Fubini once it's there.
 \end{proof}