fix eksctl getting started guide create cluster command

[bwagner5]

1. Issue, if available:
N/A

2. Description of changes:
The getting started guide eksctl create cluster command is broken.

the current guide has the following:

eksctl create cluster << 'EOF'
---
apiVersion: eksctl.io/v1alpha5
kind: ClusterConfig
metadata:
  name: ${CLUSTER_NAME}
  region: ${AWS_DEFAULT_REGION}
  version: "1.21"
  tags:
    karpenter.sh/discovery: ${CLUSTER_NAME}
managedNodeGroups:
  - instanceType: m5.large
    amiFamily: AmazonLinux2
    name: ${CLUSTER_NAME}-ng
    desiredCapacity: 1
    minSize: 1
    maxSize: 10
iam:
  withOIDC: true
EOF

if you run the above w/ --dry-run to see the generated config, the heredoc passed in does not apply and defaults are taken instead:

eksctl create cluster --dry-run << 'EOF'
---
apiVersion: eksctl.io/v1alpha5
kind: ClusterConfig
metadata:
  name: ${CLUSTER_NAME}
  region: ${AWS_DEFAULT_REGION}
  version: "1.21"
  tags:
    karpenter.sh/discovery: ${CLUSTER_NAME}
managedNodeGroups:
  - instanceType: m5.large
    amiFamily: AmazonLinux2
    name: ${CLUSTER_NAME}-ng
    desiredCapacity: 1
    minSize: 1
    maxSize: 10
iam:
  withOIDC: true
EOF

... For brevity just showing one field that verifies it is using the default randomly generated cluster name rather than the spec in the heredoc...

metadata:
  name: exciting-creature-1645053433

This PR changes it to the following which works:

eksctl create cluster --dry-run -f - << EOF
---
apiVersion: eksctl.io/v1alpha5
kind: ClusterConfig
metadata:
  name: ${CLUSTER_NAME}
  region: ${AWS_REGION}
  version: "1.21"
  tags:
    karpenter.sh/discovery: ${CLUSTER_NAME}
managedNodeGroups:
  - instanceType: m5.large
    amiFamily: AmazonLinux2
    name: ${CLUSTER_NAME}-ng
    desiredCapacity: 1
    minSize: 1
    maxSize: 10
iam:
  withOIDC: true
EOF

...again just showing metadata to verify it's picking up the spec...

metadata:
  name: my-cluster
  region: us-east-2
  tags:
    karpenter.sh/discovery: my-cluster

3. How was this change tested?

4. Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: link to issue
• No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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[spring1843]

LGTM thank you