docs(field): remove duplicate wording (#2674)

tracing-core/src/field.rs

@@ -1,8 +1,8 @@
 //! `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
-//! a `&str` but represented internally as an array index) to a [`Value`].
+//! Spans and events may be annotated with key-value data, known 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
 //!