Fix module links in std::fmt and the Rust book's documentation chapter.

The links in the rustdoc for several places in fmt were trying to link to
the std::fmt module but actually linking to std, which was confusing.
While trying to figure out why I noticed that the documentation chapter of
the Rust book has examples that show this same bug (although it doesn't seem
widespread in practice).

src/doc/trpl/documentation.md

@@ -45,7 +45,7 @@ Rust keeps track of these comments, and uses them when generating
 documentation. This is important when documenting things like enums:
 
 ```rust
-/// The `Option` type. See [the module level documentation](../) for more.
+/// The `Option` type. See [the module level documentation](index.html) for more.
 enum Option<T> {
     /// No value
     None,
@@ -57,7 +57,7 @@ enum Option<T> {
 The above works, but this does not:
 
 ```rust,ignore
-/// The `Option` type. See [the module level documentation](../) for more.
+/// The `Option` type. See [the module level documentation](index.html) for more.
 enum Option<T> {
     None, /// No value
     Some(T), /// Some value `T`