Remove GITHUB_USERNAME from the readme (rust-lang#1372)

* Remove GITHUB_USERNAME from the readme

It's no longer used.

* Improve wording

Co-authored-by: Denis Cornehl <denis@cornehl.org>

* minor wording improvements

Co-authored-by: Denis Cornehl <denis@cornehl.org>

README.md

@@ -187,12 +187,11 @@ cargo run -- build crate --local /path/to/source
 # Adds a directory into database to serve with `staticfile` crate.
 cargo run -- database add-directory <DIRECTORY> [PREFIX]
 
-# Updates github stats for crates.
-# You need to set CRATESFYI_GITHUB_USERNAME, CRATESFYI_GITHUB_ACCESSTOKEN
-# environment variables in order to run this command.
-# Set DOCSRS_GITLAB_ACCESSTOKEN to raise the rate limit,
+# Updates repository stats for crates.
+# You need to set the CRATESFYI_GITHUB_ACCESSTOKEN
+# environment variable in order to run this command.
+# Set DOCSRS_GITLAB_ACCESSTOKEN to raise the rate limit for GitLab repositories,
 # or leave it blank to fetch repositories at a slower rate.
-# You can set this environment variables in the .env file.
 cargo run -- database update-repository-fields
 ```