os/mac/diagnostic: fix case of OpenCore check

[EricFromCanada]

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nvram vars are case-sensitive.

% brew doctor check_for_opencore
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: You have booted macOS using OpenCore Legacy Patcher.
We do not provide support for this configuration.
...

[ZhongRuoyu]

Whoops, thanks!