docs: remove OpenSSL keg_only information

[cho-m]

OpenSSL is no longer keg_only since
Homebrew/homebrew-core@501a8a7

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[MikeMcQuaid]

Thanks again @cho-m!