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/libstd/primitive_docs.rs

@@ -490,7 +490,7 @@ mod prim_str { }
 /// assert_eq!(tuple.2, 'c');
 /// ```
 ///
-/// For more about tuples, see [the book](../book/primitive-types.html#Tuples).
+/// For more about tuples, see [the book](../book/primitive-types.html#tuples).
 ///
 /// # Trait implementations
 ///