r2r: add trailing slash to homepage url to avoid redirection

[luzpaz]

Homepage link https://sourceforge.net/projects/weinberg-r2r needs a trailing slash added, otherwise there's a javascript redirect.

As recommended in https://repology.org/repository/homebrew_tap_brewsci_bio/problems

• [X] Have you followed the guidelines for contributing?
• [X] Have you checked that there aren't other open pull requests for the same formula update/change?
• [ ] Have you built your formula locally with brew install --build-from-source FORMULA, where FORMULA is the name of the formula you're submitting?
• [ ] Does your build pass brew audit --strict FORMULA (after doing brew install FORMULA)?

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[YoshitakaMo]

Thank you for your contribution. Homebrew-bio is now running again. I've merged your PRs. See https://github.com/brewsci/homebrew-bio/commit/7df7bd6b025996f6f3f5788efcbbdccf5b202ad9 .