docs: fix Nushell-Homebrew setup instructions

[kimufly]

Summary

Fix the problem that ASDF_NU_DIR does not fight for an invisible newline character at the end of the brew --prefix command

[Stratus3D]

Can anyone who uses nushell test this and confirm it fixes the problem?

@kimufly can you write a unit test for this issue?

[jthegedus]

We don't really test these setup instructions in any of our CI pipelines. The Nushell tests just directly setup using Nu script instead of each permutation of Shell+asdf installation method.

If we were to add tests I guess they would live https://github.com/asdf-vm/asdf/blob/master/test/asdf_nu.bats but would have to detect OS and execute conditionally.

[jthegedus]

I'm going to merge this and we can improve the Nushell test suite over time. There are a few issues already raised in other Issues which are being addressed individually.