docs(README): update bundle command for antigen

[nesvand]

Pull request template

Type of changes

• CI
• Bugfix
• Feature
• Generic maintenance tasks
• Documentation content changes
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no URL changes)
• Improving the performance of the project (not introducing new features)
• Other (please describe):

Issue Number: #29

What is the current behavior?

Updating documentation to reflect the correct syntax to use for installing the plugin with antigen

What is the new behavior?

Nil new behaviour - prevents antigen from throwing an error on systems where main is not considered the default branch in git.

Does this introduce a breaking change?

• Yes
• No

Other information

N/A