KAFKA-19784: Unify the Id naming with other methods

Author: jiafu1115

clients/src/main/java/org/apache/kafka/clients/admin/MemberDescription.java

@@ -178,7 +178,7 @@ public Optional<String> groupInstanceId() {
     }
 
     /**
-     * The rack ID of the group member.
+     * The rack id of the group member.
      */
     public Optional<String> rackId() {
         return rackId;

clients/src/main/java/org/apache/kafka/clients/admin/ShareMemberDescription.java

@@ -77,7 +77,7 @@ public String consumerId() {
     }
 
     /**
-     * The rack ID of the group member.
+     * The rack id of the group member.
      */
     public Optional<String> rackId() {
         return rackId;