Fix broken doc link

Author: starthal

src/doc/unstable-book/src/language-features/lang-items.md

@@ -191,7 +191,7 @@ mechanisms of the compiler. This is often mapped to GCC's personality function
 which do not trigger a panic can be assured that this function is never
 called. The language item's name is `eh_personality`.
 
-[unwind]: https://github.com/rust-lang/rust/blob/master/src/libpanic_unwind/gcc.rs
+[unwind]: https://github.com/rust-lang/rust/blob/master/library/panic_unwind/src/gcc.rs
 
 The second function, `rust_begin_panic`, is also used by the failure mechanisms of the
 compiler. When a panic happens, this controls the message that's displayed on