Fix: operations -> instrutions

lectures/pdes.tex

@@ -79,7 +79,7 @@ \subsection{Atomic Instructions}
     \item {Read--Modify--Write} memory.
     \item Atomic instructions --- <<all or nothing>>.
     \item Atomic instructions are <<expensive>>.\pause
-    \item Question: Do single core systems need atomic operations?
+    \item Question: Do single core systems need atomic instructions?
   \end{itemize}
 \end{frame}