add sig ui lead emails

based on https://github.com/kubernetes/k8s.io/blob/b3ca8e41c0ed8c11407a78abf555c99f5d3ecc92/groups/sig-ui/groups.yaml#L4

Author: BenTheElder

sigs.yaml

@@ -3391,12 +3391,15 @@ sigs:
     - github: floreks
       name: Sebastian Florek
       company: Plural
+      email: [email protected]
     - github: maciaszczykm
       name: Marcin Maciaszczyk
       company: Plural
+      email: [email protected]
     - github: shu-mutou
       name: Shu Muto
       company: NEC
+      email: [email protected]
     emeritus_leads:
     - github: danielromlein
       name: Dan Romlein