Add expected failure annotation on "fixed" example to placate 'make check'.

Author: pnkfelix

doc/tutorial-borrowed-ptr.md

@@ -366,7 +366,7 @@ Things get trickier when the unique box is not uniquely owned by the
 stack frame, or when there is no way for the compiler to determine the
 box's owner. Consider a program like this:
 
-~~~
+~~~ {.xfail-test}
 struct R { g: int }
 struct S { mut f: ~R }
 fn example5a(x: @S, callback: @fn()) -> int {