README.md: replace hardcoded path to command substitution

[reishoku]

This patch replaces hardcoded path in README.md. Since Homebrew (or Linuxbrew) can be installed anywhere on user's system, use of hard-coded path should be avoided.

Signed-off-by: KOSHIKAWA Kenichi <reishoku.misc at pm.me>

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Before submitting a pull request, please make sure the following is done:

• [x] Fork the repository and create your branch from master.
• [x] If you've added code that should be tested, add tests!
• [x] Ensure the test suite passes.
• [x] Format your code with ShellCheck.
• [x] Include a human-readable description of what the pull request is trying to accomplish.
• [x] Steps for the reviewer(s) on how they can manually QA the changes.
• [x] Have a fixes #XX reference to the issue that this pull request fixes.

[inkarkat]

@karbassi Can we please tackle the huge backlogs of PRs really soon?! We're already losing some valuable contributions.