update github URLs on Installing.md

wp-cli / handbook

📖 Complete documentation for WP-CLI

https://make.wordpress.org/cli/handbook/

MIT License

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Closed ryanjkemper closed 1 year ago

ryanjkemper commented 1 year ago

At some point, it looks like the master branch was renamed to main for wp-cli/wp-cli

This PR fixes links on the Installing page.