Better fix for parsing doc

Author: Gbury

doc/parsing.md

@@ -49,7 +49,8 @@ module P =
     (Dolmen.Std.Loc)(Dolmen.Std.Id)(Dolmen.Std.Term)(Dolmen.Std.Statement)
     (Dolmen.Std.Extensions.Smtlib2)
 
-let _ = P.parse_all (`File "example.smt2")
+let _, lazy_l = P.parse_all (`File "example.smt2")
+let statements = Lazy.force lazy_l
 ```
 
 For more examples, see the [tutorial](https://github.com/Gbury/dolmen/tree/master/doc/tuto.md).