fix env_logger crate name typo in lib.rs documentation

[smacdo]

This fixes a small typo in the crate's module documentation that can mislead a first time user who is looking to use the env_logger module in tracing-log.

Motivation

A first time user following the crate.rs documentation will accidentally use env-logger as their feature flag, hit a compiler error and spend extra time trying to figure out the correct feature flag name.

Solution

Fix the typo by changing env-logger to env_logger.

[davidbarsky]

Thanks for opening this PR! You are correct to fix this, but there's a bit of unfortunate timing in that we're removing the env_logger logger feature entirely in #2772. For that reason, I'll close this PR.