message: fix(docs): Remove accidentally duplicated words
  ## Motivation
  There is a small wording mistake in the tracing-subscriber docs
  that can be fixed quickly and easily.
  ## Solution
  Two duplicate words were removed.
diff: //!
  //! ## Feature Flags
  //!
  -//! - `std`: Enables APIs that depend on the on the Rust standard library
  +//! - `std`: Enables APIs that depend on the Rust standard library
  //! (enabled by default).
  //! - `alloc`: Depend on [`liballoc`] (enabled by "std").
  //! - `env-filter`: Enables the [`EnvFilter`] type, which implements filtering
  