Remove deprecated mutable type constructor from run-pass/issue-511

This was triggering a warning that was making the pretty-print test fail

Author: brson

src/test/run-pass/issue-511.rs

@@ -1,7 +1,7 @@
 use std;
 import std::option;
 
-fn f<@T>(&o: mutable option::t<T>) {
+fn f<@T>(&o: option::t<T>) {
     assert o == option::none;
 }