README: master -> main

[osalbahr]

Resolve " Branch master was renamed to main. " when opening the url.

[stellarhopper]

@osalbahr This looks good - I'll apply manually because of signoffs, and I plan to tweak the commit message to read something like:

README: Fix a URL renaming 'master' to 'main'

With 'master' in the URL, github throws a warning:

  Branch master was renamed to main.

when opening the url. Replace it with 'main' to fix this.

I do need a Signed-off-by line from you - see https://github.com/pmem/ndctl/blob/main/CONTRIBUTING.md (number 6) for more details.