[eacsl] Add support for "admit" in reference manual

src/plugins/e-acsl/doc/refman/changes_modern.tex

@@ -157,6 +157,7 @@ in \lstinline|\\at|}
 \begin{itemize}
 \item \changeinsection{expressions}{mark logic function and predicate
   applications as implemented}
+\item \changeinsection{fn-behavior}{mark admit clauses as implemented}
 \end{itemize}
 
 \subsection*{Version Titanium-22}

src/plugins/e-acsl/doc/refman/fn_behavior.tex

@@ -3,7 +3,7 @@
                             { decreases-clause? } simple-clause*
                             named-behavior* completeness-clause*
   \
-  clause-kind ::= "check" | { "admit" }
+  clause-kind ::= "check" | "admit"
   \
   requires-clause ::= clause-kind? "requires" pred ";"
   \