[stack.cons] Fix typo in constructor parameter name

Fix parameter that should have been "cont" instead of "const". Thanks
to Daniel Krügler!

Author: Stefanus Du Toit

source/containers.tex

@@ -8483,7 +8483,7 @@
 \end{itemdescr}
 
 \begin{itemdecl}
-explicit stack(Container&& const = Container());
+explicit stack(Container&& cont = Container());
 \end{itemdecl}
 
 \begin{itemdescr}