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>

---

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

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

[karbassi]

Thank you for a great PR. 👍

[inkarkat]

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