tex: highlight fmt keywords

tex/part-practical.tex

@@ -311,6 +311,7 @@ addition to anything the base \texttt{slog.Logger} offers.
 \begin{lstlisting}[language=Go, caption={Example formatting expression supplied
 to the logger}, label=goFmtExpression, basicstyle=\linespread{0.9}\small\ttfamily,
 backgroundcolor=\color{lstbg},
+otherkeywords={\%s, \%q, \%v},
 ]
 slogger.Debugf("operation %q for user %q completed at %s", op, usr.ID, time.Now())
 \end{lstlisting}