Fixing 404 link in README.md

[nstielau]

Fixing link to non-existant use-cases.md file, pointing to about.md instead, which has a use-cases section. Using relative link, rather than full URL to the specific use-case section: https://github.com/converged-computing/oras-csi/blob/main/docs/about.md#use-cases

[vsoch]

Thank you! Likely it was originally named use-cases.md, and I renamed when I wrote the rendered docs.

[vsoch]

@nstielau could you please rebase? Apologies for the break / delay - it was a test that seems to work locally but not in the environment here - need to look into it further.