Fix unclear line of the manual (\cventry optional/empty arguments)

manual/moderncv_userguide.tex

@@ -283,7 +283,7 @@ Inside sections, you can choose between different kind of items depending on the
 {optional: comment/job description}
   \end{verbatim} 
 \vspace{-20pt}  
-where the last three arguments are optional
+where the last three arguments can be left empty
  \item inside `language' cvsection environment, you can type every entry with:
  \begin{verbatim}
 \cvlanguage{name}{level}{comment}