Change default council-docs repository branch to main
    
    This commit corresponds to Fedora-Council/council-docs#93:
    
        https://pagure.io/Fedora-Council/council-docs/pull-request/93
    
    The default git branch is renamed from `master` to `main` for the
    council-docs repo. This closes a long-standing issue on the council-docs
    repo. This commit should be merged in close coordination with
    council-docs#93.
    
    Signed-off-by: Justin W. Flory (he/him) <git@jwf.io>