message: Fix env_logger feature name in README
  ## Motivation
  I stumbled upon this issue when trying to get tracing-log working with
  env_logger. The comments in-code are right, but the README uses a hyphen
  instead of an underscore.
  ## Solution
  Switched the hyphen for an underscore :)
diff: @@ -41,7 +41,7 @@ This crate provides:
  - [`LogTracer`], a [`log::Log`] implementation that consumes [`log::Record`]s
  and outputs them as [`tracing::Event`]s.
  - An [`env_logger`] module, with helpers for using the [`env_logger` crate]
  - with `tracing` (optional, enabled by the `env-logger` feature).
  + with `tracing` (optional, enabled by the `env_logger` feature).
  [`tracing`]: https://crates.io/crates/tracing
  [`log`]: https://crates.io/crates/log
  