message: subscriber: update tracing_subscriber::fmt docs
  Brings the `fmt` docs in line with the crate docs.
diff: //!
  //! ```toml
  //! [dependencies]
  -//! tracing-subscriber = "0.2"
  +//! tracing-subscriber = "0.3"
  //! ```
  //!
  -//! *Compiler support: requires rustc 1.39+*
  +//! *Compiler support: [requires `rustc` 1.42+][msrv]*
  +//!
  +//! [msrv]: ../index.html#supported-rust-versions
  //!
  //! Add the following to your executable to initialize the default collector:
  //! ```rust
  