add missing \ on page 156

Author: oetiker

src/spec.tex

@@ -114,7 +114,7 @@ \section{Indexing}\label{sec:indexing}
 \end{example}
 \end{chktexignore}
 
-Note that the texttt{makeindex} has no clue about characters outside the ASCII range. To
+Note that the \texttt{makeindex} has no clue about characters outside the ASCII range. To
 get the sorting correct, use the \verb|@| character as shown in the K\"ase
 and \'ecole examples above.