[update] documentation

doc/tutorial/tutorial.tex

@@ -66,7 +66,10 @@ git clone https://github.com/texane/stlink stlink.git
 The GDB server is called st-util and is built using:\\
 \begin{small}
 \begin{lstlisting}[frame=tb]
-$> cd stlink.git; make ;
+$> cd stlink.git;
+$> make ;
+$> cd gdbserver ;
+$> make ;
 \end{lstlisting}
 \end{small}