clarify behavior of github_access_token option

[InternetUnexplorer]

I got distracted :( so this also fixes a typo and makes some other tiny changes that I noticed when I was looking at the README. I can remove those changes or move them to another commit if you'd like. The main difference in content is:

Before

• github_access_token: configure nix to pull from github using the given github token. This helps work around rate limit issues.

After

• github_access_token: configure Nix to pull from GitHub using the given GitHub token. This helps work around rate limit issues. Has no effect when access-tokens is also specified in extra_nix_config.

This should hopefully make the option's behavior clear without being verbose, although I'm open to other ideas as well :)