prefix.sh: repair style

[hyuraku]

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repair prefix.sh style for brew style with shellcheck --enable=all

brew/Library/Homebrew/style.rb

Line 188 in fa3af8d

# TODO: Add `--enable=all` to check for more problems.

related #11086

[MikeMcQuaid]

Suggested change

	# These variables is set by brew.sh
	# HOMEBREW_PREFIX and HOMEBREW_REPOSITORY are set by brew.sh

[MikeMcQuaid]

Thanks again @hyuraku!