Fixing 404 link in README.md

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

README.md

@@ -1,7 +1,7 @@
 # CSI ORAS Driver
 
 This repository is a test to create a CSI driver into one that uses [ORAS](https://oras.land) to
-generate a cluster-level cache of artifacts. Read about the [use cases](docs/use-cases.md) or jump in!
+generate a cluster-level cache of artifacts. Read about the [use cases](docs/about.md) or jump in!
 
 ```console
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