Fix outdated utypedtree.fsx

Author: Krzysztof-Cieslak

fcs/docsrc/content/untypedtree.fsx

@@ -92,7 +92,7 @@ code](https://github.com/fsharp/fsharp/blob/master/src/fsharp/ast.fs#L464).
 
 The relevant parts are in the following namespace:
 *)
-open FSharp.Compiler.Ast
+open FSharp.Compiler.SyntaxTree
 (**
 
 When processing the AST, you will typically write a number of mutually recursive functions