agda: Fix the post-install suggestions

[mayaofspring]

The default agda app dir to write the libraries to is ~/.agda, not ~/.config/agda as the existing version suggested. I got burned by that so now I'm fixing it

https://agda.readthedocs.io/en/v2.6.4.3/getting-started/installation.html#os-x

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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