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

@@ -272,6 +272,8 @@ 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

@@ -1025,6 +1025,8 @@ 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

@@ -80,7 +80,7 @@
 //! 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