Replace # with x in the definition of algebraic elements

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/math/realclosure/realclosure.cpp

@@ -5797,7 +5797,7 @@ namespace realclosure {
 
         struct display_free_var_proc {
             void operator()(std::ostream & out, bool compact) const {
-                out << "#";
+                out << "x";
             }
         };