[Doc] update copyright year in userman

doc/userman/userman.tex

@@ -29,7 +29,7 @@
 \end{tabular}
 \vfill
 \begin{flushleft}
-  \textcopyright 2009-2020 CEA LIST
+  \textcopyright 2009-2021 CEA LIST
 
 This work has been supported by the ANR project CAT (ANR-05-RNTL-00301) and by
 the ANR project U3CAT (08-SEGI-021-01).