fix: Dead link in 21.1

Fix #360

Author: ktgw0316

src/content/3.5/monads-and-effects.tex

@@ -12,9 +12,10 @@
 \section{The Problem}
 
 Here is a short list of similar problems, copied from
-\urlref{https://core.ac.uk/download/pdf/21173011.pdf}{Eugenio Moggi's
-  seminal paper}, all of which are traditionally solved by abandoning the
-purity of functions.
+Eugenio Moggi's seminal paper,\footnote{Eugenio Moggi:
+  \href{https://doi.org/10.1016/0890-5401(91)90052-4}{
+    Notions of Computation and Monads}. Inf. Comput. 93(1): 55-92 (1991).}
+all of which are traditionally solved by abandoning the purity of functions.
 
 \begin{itemize}
   \tightlist