Update antigen install instructions.

[unixorn]

Description

Antigen doesn't cope when the default branch of a repo is not named master - see zsh-users/antigen#717

Update the install instructions in the readme accordingly.

Type of changes

• Add/update a helper script or git alias
• Add/update link to an external resource like a blog post or video
• Text cleanups/updates
• Test updates

License Acceptance

• This repository is Apache version 2.0 licensed (some scripts may have alternate licensing inline in their code) and by making this PR, I am contributing my changes to the repository under the terms of the Apache 2 license.

Checklist

• I have read the CONTRIBUTING document.
• All new and existing tests pass.
• Any scripts added use #!/usr/bin/env interpreter instead of potentially platform-specific direct paths (#!/bin/sh is an allowed exception)
• Scripts added/updated in this PR are all marked executable.
• Scripts added/updated in this PR do not have a language file extension unless they are meant to be sourced and not run standalone. No one should have to know if a script was written in bash, python, ruby or whatever. Not including file extensions makes it easier to rewrite the script in another language later without having to change every reference to the previous version.
• I have added a credit line to README.md for any new scripts.
• If there was no author credit inside a script added in this PR, I have added one.
• I have confirmed that the link(s) in my PR are valid.

[codeclimate]

Code Climate has analyzed commit 3ab9de6 and detected 0 issues on this pull request.

View more on Code Climate.

[github-actions]

🦙 MegaLinter status: ⚠️ WARNING

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ COPYPASTE	jscpd	yes		no	1.01s
⚠️ MARKDOWN	markdownlint	1		1	0.37s
⚠️ MARKDOWN	markdown-table-formatter	1		1	0.37s
✅ REPOSITORY	devskim	yes		no	0.57s
✅ REPOSITORY	dustilock	yes		no	0.01s
✅ REPOSITORY	gitleaks	yes		no	0.31s
✅ REPOSITORY	git_diff	yes		no	0.04s
✅ REPOSITORY	secretlint	yes		no	2.3s
✅ REPOSITORY	syft	yes		no	0.38s
✅ REPOSITORY	trivy	yes		no	2.62s
✅ SPELL	misspell	1		0	0.04s

See errors details in artifact MegaLinter reports on CI Job page
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