fix dead cosign quickstart URL (#81)

README.md

@@ -39,7 +39,7 @@ The user experience is much nicer if you [set use Ptyxis](https://flathub.org/ap
 
 ## Verification
 
-These images are signed with sisgstore's [cosign](https://docs.sigstore.dev/cosign/overview/). You can verify the signature by downloading the `cosign.pub` key from this repo and running the following command:
+These images are signed with sisgstore's [cosign](https://docs.sigstore.dev/quickstart/quickstart-cosign/). You can verify the signature by downloading the `cosign.pub` key from this repo and running the following command:
 
     cosign verify --key cosign.pub ghcr.io/ublue-os/boxkit