Help users avoid specifying URL scheme an path

We’ve noticed that some users still include the URL scheme and full path
when logging into an OCI registry, for example:

```sh
helm registry login -u $OCI_REGISTRY_USER --password-stdin oci://ghcr.io/org/repo
```

This is no longer necessary and will not be supported in Helm v4.

To guide users toward the correct usage, we should show an example of
the ideal command.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

pkg/cmd/registry_login.go

@@ -33,6 +33,10 @@ import (
 
 const registryLoginDesc = `
 Authenticate to a remote registry.
+
+For example for Github Container Registry:
+
+    echo "$GITHUB_TOKEN" | helm registry login ghcr.io -u $GITHUB_USER --password-stdin
 `
 
 type registryLoginOptions struct {