Revert "Fix up links"

This reverts commit 7f1d1c6d9a.

The original commit was created because mdBook and rustdoc had
different generation algorithms for header links; now with
https://github.com/rust-lang/rust/pull/39966 , the algorithms
are the same. So let's undo this change.

... when I came across this problem, I said "eh, this isn't fun,
but it doesn't take that long." I probably should have just actually
taken the time to fix upstream, given that they were amenable. Oh
well!

src/liballoc/arc.rs

@@ -102,7 +102,7 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 /// [downgrade]: struct.Arc.html#method.downgrade
 /// [upgrade]: struct.Weak.html#method.upgrade
 /// [`None`]: ../../std/option/enum.Option.html#variant.None
-/// [assoc]: ../../book/method-syntax.html#Associated%20functions
+/// [assoc]: ../../book/method-syntax.html#associated-functions
 ///
 /// # Examples
 ///

src/liballoc/rc.rs

@@ -215,7 +215,7 @@
 //! [downgrade]: struct.Rc.html#method.downgrade
 //! [upgrade]: struct.Weak.html#method.upgrade
 //! [`None`]: ../../std/option/enum.Option.html#variant.None
-//! [assoc]: ../../book/method-syntax.html#Associated%20functions
+//! [assoc]: ../../book/method-syntax.html#associated-functions
 //! [mutability]: ../../std/cell/index.html#introducing-mutability-inside-of-something-immutable
 
 #![stable(feature = "rust1", since = "1.0.0")]