Fix typo in preface

src/preface.tex

@@ -6,7 +6,7 @@ \section*{Preface}
 
 \begin{itemize}
     \item \textit{Bertsimas, Dimitris, and John N. Tsitsiklis. Introduction to linear optimization. Vol. 6. Belmont, MA: Athena Scientific, 1997.}
-    \item \textit{Wolsey, Laurence A. Integer programming. John Wiley & Sons, 2020.}
+    \item \textit{Wolsey, Laurence A. Integer programming. John Wiley \& Sons, 2020.}
 \end{itemize}
 
 Among the years, errors and typos have been found and introduced as the notes evolve. It is likely that some still exists. If so please feel free to open an issue in the its github repository and report it or make a pull request proposing a fix.