[doc][handbook] fix underscore interpretation

doc/handbook/4_developingdumux.tex

@@ -39,7 +39,7 @@ more information.
 \paragraph{Optimized computation vs debugging}
 \Dune and \Dumux are built with the help of \texttt{dunecontrol}, as explained on page \pageref{buildIt}.
 Per default, \Dumux is compiled using optimization options, which leads to faster runtimes but is unsuitable
-for debugging. For debug opts you can set \texttt{DCMAKE_BUILD_TYPE} to \texttt{Debug} or \texttt{RelWithDebInfo}
+for debugging. For debug opts you can set \texttt{DCMAKE\textunderscore BUILD\textunderscore TYPE} to \texttt{Debug} or \texttt{RelWithDebInfo}
 in your options file. You can also do this in any of the \texttt{CMakeLists.txt} in Dumux by adding:
 
 \begin{lstlisting}[style=Shell]