[e-acsl] fix userman's foreword

src/plugins/e-acsl/doc/userman/biblio.bib

+@article{baudin21cacm,
+  author = {Baudin, Patrick and Bobot, Fran\c{c}ois and B\"uhler, David and Correnson, Lo\"ic and Kirchner, Florent and Kosmatov, Nikolai and Maroneze, Andr\'e and Perrelle, Valentin and Prevosto, Virgile and Signoles, Julien and Williams, Nicky},
+  title = {{The Dogged Pursuit of Bug-Free C Programs: The Frama-C Software
+              Analysis Platform}},
+  journal = {Communications of the ACM},
+  year = 2021,
+  month = aug,
+}
+
 @manual{userman,
   title  = {Frama-C User Manual},
   author = {Loïc Correnson and Pascal Cuoq and Florent Kirchner and

src/plugins/e-acsl/doc/userman/main.tex

@@ -40,7 +40,7 @@
 This is the user manual of the \framac plug-in
 \eacsl\footnote{\url{https://frama-c.com/fc-plugins/e-acsl.html}}. The contents
 of this document correspond to its version \eacslpluginversion compatible with
-\fcversion version of \framac~\cite{userman,fac15}.  The development of
+\fcversion{} version of \framac~\cite{userman,baudin21cacm}. The development of
 the \eacsl plug-in is still ongoing.  Features described by this document may
 evolve in the future.