subscriber: fix a few broken RustDoc links (#1701)

This fixes a handful of broken RustDoc links I noticed in
`tracing-subscriber`.

tracing-subscriber/src/fmt/format/mod.rs

@@ -229,6 +229,8 @@ where
 ///
 /// Additionally, a `Writer` may expose additional `tracing`-specific
 /// information to the formatter implementation.
+///
+/// [fields]: tracing_core::field
 pub struct Writer<'writer> {
     writer: &'writer mut dyn fmt::Write,
     // TODO(eliza): add ANSI support

tracing-subscriber/src/fmt/writer.rs

@@ -988,6 +988,8 @@ impl<A, B> Tee<A, B> {
     /// outputs.
     ///
     /// See the documentation for [`MakeWriterExt::and`] for details.
+    ///
+    /// [writers]: std::io::Write
     pub fn new(a: A, b: B) -> Self {
         Self { a, b }
     }

tracing-subscriber/src/lib.rs

@@ -56,7 +56,7 @@
 //! used without requiring the Rust standard library, although some features are
 //! disabled. Although most of the APIs provided by `tracing-subscriber`, such
 //! as [`fmt`] and [`EnvFilter`], require the standard library, some
-//! functionality, such as the [`Subscriber`] trait, can still be used in
+//! functionality, such as the [`Subscribe`] trait, can still be used in
 //! `no_std` environments.
 //!
 //! The dependency on the standard library is controlled by two crate feature
@@ -102,6 +102,7 @@
 //! [`parking_lot`]: https://crates.io/crates/parking_lot
 //! [`time` crate]: https://crates.io/crates/time
 //! [`liballoc`]: https://doc.rust-lang.org/alloc/index.html
+//! [`libstd`]: https://doc.rust-lang.org/std/index.html
 #![doc(html_root_url = "https://docs.rs/tracing-subscriber/0.2.12")]
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png",