docs: update Homebrew install command in README

[1337Core]

What’s changed
• README – swapped the old, deprecated command

brew cask install font-lexend-deca

for the current syntax:

brew install --cask font-lexend-deca

Why

brew cask was removed from Homebrew; the new consolidated syntax avoids the runtime error:

Error: brew cask is no longer a brew command. Use brew <command> --cask instead.

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