[BUG] Documentation Fix in Readme for "leader_cluster"

[Rishikesh1159]

What is the bug?
In Readme file Step 4 if we use "leader_cluster" it throws following error:

curl -XPUT "http://${FOLLOWER}/_plugins/_replication/follower-01/_start?pretty" -H 'Content-type: application/json' -d'{"leader_cluster":"leader-cluster", "leader_index": "leader-01"}'
{
  "error" : {
    "root_cause" : [
      {
        "type" : "x_content_parse_exception",
        "reason" : "[1:2] [FollowIndexRequestParser] unknown field [leader_cluster] did you mean any of [leader_alias, leader_index]?"
      }
    ],
    "type" : "x_content_parse_exception",
    "reason" : "[1:2] [FollowIndexRequestParser] unknown field [leader_cluster] did you mean any of [leader_alias, leader_index]?"
  },
  "status" : 400
}

How can one reproduce the bug?
Steps to reproduce the behavior:
Follow steps in Getting Started

What is the expected behavior?
Replace "leader_cluster" with "leader_alias"