[typo] [doc] fix typo.

doc/buildsystem/buildsystem.tex

@@ -1311,7 +1311,7 @@ entries in the opts file:
 \item \texttt{BUILDDIR} configure the modules to use out-of-source
   build. The binaries are built in subdirectories of each module,
   named according to the \texttt{BUILDDIR} variable.
-  If \texttt{BUILDDIR} is an absolute path (starts with \texttt{'\string\'}),
+  If \texttt{BUILDDIR} is an absolute path (starts with \texttt{'\string/'}),
   the build directory is set to \texttt{BUILDDIR/module-name} for each module.
 \item \texttt{\textit{COMMAND}\,\_FLAGS}\\ the most important ones are:
   \begin{itemize}