Fix README homebrew install regarding glfw3

[oksome]

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[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3088

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[metajack]

I'm confused by this. We don't actually need glfw3 installed since glfw-rs dpeends on glfw-sys which provides one.

[metajack]

Perhaps just remove glfw3 from the list of dependencies?

[oksome]

I have just tested uninstalling glfw3, and the build still appears to work.

[jdm]

@oksome: Want to make this PR remove the dependency, in that case?

[jdm]

Fixed by #3991.