fix(documentation): use secret key referred in eventsource

[vranystepan]

apiVersion: v1
kind: Secret
metadata:
  name: github-access
type: Opaque
data:
  access: <base64-encoded-api-token-from-previous-step>

vs.

...
      # apiToken refers to K8s secret that stores the github api token
      apiToken:
        # Name of the K8s secret that contains the access token
        name: github-access
        # Key within the K8s secret whose corresponding value (must be base64 encoded) is access token
        key: token
...

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[VaibhavPage]

lgtm. Thanks!