Broken link to ASDF_CONFIG_FILE environment variable

[lindhe]

How can we improve the documentation?

Here, the docs refer to the ASDF_CONFIG_FILE environment variable:

asdf/docs/manage/configuration.md

Line 57 in ccdd47d

`${HOME}/.asdfrc` is the default location used by asdf. This can be set with the [Environment Variable `ASDF_CONFIG_FILE`](#asdfconfigfile).

That link is broken. On GitHub, the correct link seems to be #asdf_config_file while on the docs site it seems to be #asdf-config-file.

Considering the discrepancy, maybe it's best to just link to #environment-variables which seems to work the same on the web page and on GitHub?