Update git url for generate_readme workflow

[hisaac]

Checklist

• I've read and followed the Contribution Guidelines
• step.yml and README.md is updated with the changes (if needed)

Version

Requires a MAJOR/MINOR/PATCH version update

We have yet to do an official release, but will do that after this is merged.

Context

For a reasons I'm not aware of, the steps-readme-generator Step needs to have its main branch specified in the git URL in the bitrise.yml file. If it's omitted, then it tries to load a master branch, which is nonexistent. This is a known issue, so hopefully will be addressed at some point.

Changes

• Adds @main to the end of the git URL for the generate_readme workflow.