docs/Adding-Software-to-Homebrew: remove braces

[cho-m]

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Avoid template handling which results in empty output.

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Snippets of https://docs.brew.sh/Adding-Software-to-Homebrew#pushing for previous output:

git push  my-new-cask-branch

remote:      https://github.com//homebrew-cask/pull/new/my-new-cask-branch

[Copilot]

Pull request overview

Updates the “Adding Software to Homebrew” contributor documentation to avoid {{...}} template-style placeholders that can render as empty output on the published docs site.

Changes:

• Replace {{my-github-username}} placeholders with plain my-github-username in the git push example and the resulting GitHub URL example.

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

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