KEP-3619: fix typo: "RutimeClass" -> "RuntimeClass"

Author: everpeace

keps/sig-node/3619-supplemental-groups-policy/README.md

@@ -1228,7 +1228,7 @@ N/A
 
 ## Alternatives
 
-### Introducing `RutimeClass`
+### Introducing `RuntimeClass`
 
 As described in the [Motivation](#motivation) section, cluster administrators would need to deploy a custom low-level container runtime(e.g., [pfnet-research/strict-supplementalgroups-container-runtime](https://github.com/pfnet-research/strict-supplementalgroups-container-runtime)) that modifies OCI container runtime spec(`config.json`) produced by CRI implementations (e.g., containerd, cri-o). A custom `RuntimeClass` would be introduced for it.