tracing: fix docs typos and warnings(tokio-rs#2581)

## Motivation

When reading the docs I noticed some typo's and while fixing them I
noticed warnings from cargo doc

## Solution

Fixed the typos and the doc warnings in tracing

Co-authored-by: Neulichedl, Patrick - D0E05450 <Patrick.Neulichedl@dm-b2b.com>

tracing/src/field.rs

@@ -1,13 +1,13 @@
 //! `Span` and `Event` key-value data.
 //!
-//! Spans and events may be annotated with key-value data, referred to as known
-//! as _fields_. These fields consist of a mapping from a key (corresponding to
+//! Spans and events may be annotated with key-value data, referred to as _fields_.
+//! These fields consist of a mapping from a key (corresponding to
 //! a `&str` but represented internally as an array index) to a [`Value`].
 //!
 //! # `Value`s and `Subscriber`s
 //!
 //! `Subscriber`s consume `Value`s as fields attached to [span]s or [`Event`]s.
-//! The set of field keys on a given span or is defined on its [`Metadata`].
+//! The set of field keys on a given span or event is defined on its [`Metadata`].
 //! When a span is created, it provides [`Attributes`] to the `Subscriber`'s
 //! [`new_span`] method, containing any fields whose values were provided when
 //! the span was created; and may call the `Subscriber`'s [`record`] method
@@ -126,7 +126,6 @@ use crate::Metadata;
 /// should be used whenever possible.
 ///
 /// </pre>
-/// </div>
 pub trait AsField: crate::sealed::Sealed {
     /// Attempts to convert `&self` into a `Field` with the specified `metadata`.
     ///

tracing/src/span.rs

@@ -840,7 +840,7 @@ impl Span {
     ///
     /// <pre class="ignore" style="white-space:normal;font:inherit;">
     ///     <strong>Note</strong>: The returned <a href="../struct.EnteredSpan.html">
-    ///     <code>EnteredSpan</a></code> guard does not implement <code>Send</code>.
+    ///     <code>EnteredSpan</code></a> guard does not implement <code>Send</code>.
     ///     Dropping the guard will exit <em>this</em> span, and if the guard is sent
     ///     to another thread and dropped there, that thread may never have entered
     ///     this span. Thus, <code>EnteredSpan</code>s should not be sent between threads.