texmath: correct license

[carlocab]

See https://github.com/jgm/texmath/search?q=license, #67169

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• Have you followed the guidelines for contributing?
• 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?
• Is your test running fine brew test <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>)?

[carlocab]

@chenrui333 fixed the license. Maybe this doesn't need a full CI run.

[chenrui333]

LGTM